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Hero 1 goes through the gauntlet to become a hero, and it's left him bitter. He's angry at how unfair it was. How many good men died just to prove purity (or whatever). He is a hero, but he's broken.

Hero 2 goes through the gauntlet to follow in his idol's footsteps: Hero 1. He gets through the gauntlet because he keeps reminding himself about Hero 1. He's got a symbol of hope.

However you tease the "twist" to the reader that this is not going to go as planned is up to you.

Hero 2 can hear bad things about his idol and deny it. Hero 2 has to put Hero 1 on a pedestal to preserve the symbol of hope, so he willingly ignores a few bad things that the reader knows is objectively true.

Meanwhile Hero 1 has been observing Hero 2's progress and has mixed emotions. He's probably going to die a meaningless death like all the others, or emerge jaded and broken just like himself. But as he comes closer to finishing, Hero 1 is emotionally invested, maybe contemplates cheating, intervening for Hero 2…, or is he suppose to stand back dispassionately? While Hero 1 is debating it (he's a broken hero), Hero 2 sees "a vision" of Hero 1 watching over him. Hero 1 is like "oh whatever, just die" and leaves, but Hero 2 is like "Hero 1 is my guardian angel! He watched over me and knew I could make it!"

By the time Hero 2 finishes the gauntlet, the reader knows these guys have very different ideologies, and those ideologies can't survive meeting each other. 2/3rds of the novel they finally meet and readers are excited, because they are in on the "twist", but the two characters aren't.

The last 3rd of the story is how this is reconciled. Hero 2 is disillusioned when his ideology falls, but it was flawed to begin with. He's got to find his own strength, not just idol worship. Meanwhile Hero 1 has either created a self-fulfilled prophecy that the gauntlet is a bad hero-maker, or he realizes that a hero is more than winning challenges it is a symbol of hope and he has a path to redemption.

You still get the twist but it's not about tricking the reader, it's about tricking the characters.

Probably opinion-based, but consider the goal is not to "trick the reader" but to put your MC in agony and conflict. Everything they worked for and believed in up to that point is stripped away! The reader is turning pages because they know that a big reveal is coming, and they can't wait to watch this collision. Let your reader in on the twist (by stages), don't tell the characters it's coming. You get suspense which is a better build, and there's still a twist because the characters will do SOMEthing – we have no idea what – but it's more about wrecking these characters' lives.

Have you ever played the game Diablo? In it, a knight comes to battle evil. A young prince has been possessed by a demon and much needs setting right in the realm.

At the end, when the player has finally defeated Diablo, there is a cut scene where we see the prince freed from the demon and our character is holding a stone. This stone is the essence of evil and is powerful. Our character succumbs to temptation and becomes the next evil to be fought in D2 & D3.

It was a shock, but worked.

I think you should gradually move your hero into darkness and let the deciding moment be something that, while unthinkable, is necessary for the defeat of evil.

Another example comes to mind. In 24, Jack Bauer went from an honest, by the rules CTU agent with a desperate problem to someone who understood the necessity of evil when you are fighting the good fight.

In Falling Down, the Micheal Douglas character was unaware that he had become a villain - he was just trying to get to see his daughter.

The timing might be a little early. It depends on your understanding of hero one - was his corruption slow and gradual or a quick slide down the slope? If it took time, give it time.

There's no such thing as "too early in the story for a plot twist". There's even a trope called "First-Episode Spoiler", for when the very beginning of a story contains a plot twist that's pivotal to the rest of the story (obligatory TV Tropes warning).

In your case, where you've got the parallel journeys, you could write them in such a way that the first hero seems virtuous and strong-willed up until his corruption, but once you read the second hero's journey, you realise just how much more virtuous and strong-willed he is in comparison. This way the first hero's journey will still come as a shock to most readers, but as they read on, they'll realise in hindsight that it's actually not all that unexpected.

I would foreshadow the fundamental corruptibility of the hero first. He cheats at a game, gets caught, and is unrepentant. He chases and stops the robber of a rich man -- then demands half the loot to let him go, and returns to tell the victim the robber escaped him.

But at the same time, I would show him, perhaps interleaved with those scenes, actually altruistic and brave. He saves a girl about to be raped, sends the criminals packing (or injured or dead by his battle prowess), with no reward.

Part of the money he took from the robber he hands out to an old homeless woman, again just to help, without reward or exchange.

If throughout his journey you make him a mixed bag of morality, both self-interested and greedy but also moved to sympathy and altruism for the helpless, you can make sure the reader isn't certain exactly which side will surface, or prevail.

If possible, I would ensure he accomplishes his altruistic mission to defeat an evil that persecutes some innocent people, but once that is done, there is just too much wealth and power in the villain's lair for him to just walk away from it.

One option is to move the twist even earlier in your story! If you want to focus primarily on Hero 2, compress Hero 1's journey into just the prologue. Not necessarily literally the first chapter, but the part of the story to set up your world before the inciting incident on Hero 2's journey.

The risk with waiting until a third of the way through the story to drop your twist is that your readers will become emotionally invested in Hero 1 and expect to go through the entire story with them. Then, well after your story is underway, you're taking Hero 1 away from your readers and suddenly asking them to follow along with an entirely different character. Pulling this switch off in the middle of a story is difficult and can easily alienate your readers.

However, dropping the twist at the beginning of your work avoids this problem. Your readers will still be looking for the main thread and won't feel betrayed by a short story to set up the main arc before meeting the main character. And a twist like Hero 1 becoming corrupted works well for this kind of short vignette. The best short stories have a twist ending, and in the context of a broader story, a prologue needs to ask a compelling question. Going on a short journey with a heroic character only for them to turn out to be evil accomplishes both of those goals wonderfully.

The goal of your story is to engage the reader. Getting them invested in the MC is a good way to do that, but when the MC turns villain, readers might feel tricked or betrayed and that is something you don't want.

To avoid that, you can use a lot of foreshadowing so that the reader is "in on it" and only the MC himself is surprised by how things turn out in the end. But, of course, this takes a lot of drama out of the whole story. Drama and tension that you probably want in the story.

One thing you can do is to overlap your timelines and make the reader invested in your second MC before the twist. He could appear as a minor character at first and then grow into becoming the MC after the first one fades into the background.

This is essentially how Game of Thrones does it, just on a larger scale. They kill off MCs all the time, but there are always other characters already standing by to take the spotlight.

Well, in Star Wars, the "hero" of episode 1 to 3 turns into the villain in movie 3 out of 9 so thats about on-par with your timetable.

The only thing is that they didnt start at the first episode.
You might be able to pull something off by introducing the characters at the same time in the book but not letting the reader know they are actually at different points in world-time.

There is no "too soon" or "to late" for a plot twist. It's important how it's executed. They way I would try to lead to this twist is to keep him shown as a good guy. In many stories, "good" guys kill "bad" guys. But how do you know who's the good and who's the bad person? Normally this is shown by the actions of those persons to neutral characters and towards each other. If you write from the point of the "hero" you can portray his deeds as good, as he thinks they're good and rightouss But if you slowly zoom out of the view of the "hero" you can lead the reader to more and more doubt on his actions.

This will give the reader the ability to foresee a plot-twist, at least if he pays enough attention to the hints, and it gives really good approach to the question "what is good, what makes a hero?"

Just an idea but what's to stop you from writing things out of chronological order?

The initial hero may turn villain 1/3 the way through chronologically but you could choose to write two distinct time lines, one following the initial hero's decent into villainy and one following the second hero where this time line will intercept at the climactic moment the former and latter heroes meet.

By Separating the time lines and skewing them you could have the plot twist happen at just about any place in your story.

One-third of the way in is an excellent place for a plot twist, but a bad place to switch viewpoints

Using a plot twist to catapult your story from its first act to its second works wonderfully. It gets readers attention, and sets a path for the rest of the story to take that was not one that they were expecting. It allows you to take the characters that the readers have come to know and love, and put them in situations that they were not expecting themselves to be in.

On the other hand, this is a bad place to switch viewpoints. The first act is all about introducing the characters, getting to know them and care about them. Switching to a new viewpoint at this point in the story undercuts all of that, and will frustrate your reader, who will be more interested in following the characters you've introduced so far (presumably you've made them interesting and engaging, yes?) than starting all over with new ones.

Having said that, there are some stories that tell the first third or so from a different viewpoint than the eventual main character. The Passage by Justin Cronin does this, as does (to a slightly lesser extent), Santa Olivia by Jacqueline Carey. The trick is that in each of these cases, the trick works by strongly signaling that the initial viewpoint will not be the main one, and by finishing the narrative arcs of the initial viewpoints.

Those techniques run counter to your goal of creating a surprising reveal, and will likely to be hard to pull off in the story you're trying to tell.

If you haven't read Mistborn by Brandon Sanderson, I heartily recommend it. It too features a hero turned villainous, but it does so in the prehistory of the story, and lets the details unfold through exposition and storytelling rather than making it part of the plot of the book itself.

I don't think you should leave the twist out, though it might be a little too soon to incorporate twist. You also should not make it clear from the beginning that the hero is corrupt, instead showing it little by little.

No, not at all... It's the plot twists that keep the readers engaged to the story. Show him good in the start. Then, as the story unravels, show what makes him go corrupt, and describe his transition from the good to bad side slowly, chapter by chapter. And use the climax or the last fight to show that bad, corrupted side of him completely.

Yet another option is to follow Hero2 from the start. He follows in Hero1’s footsteps, hears stories about what exactly happened here, sometimes does the same, sometimes acts differently. He correctly guesses that if Hero1 has successfully surmounted this obstacle, then the magical artifact used to do it must lie nearby. Possibly he recharges it, uses it and then carries it back where it belongs, performing a part of Hero1’s journey in reverse. Anyway he gets to the point where Hero1 was last seen alive, and moves on, trying to guess what Hero1 would do in his situations and how far he had gone, fearing that at any moment he’s going to encounter his tomb and be left without guidance, instead having to make his own decisions. And when Hero2, having developed way beyond the shadow of Hero1 that he was in the beginning, finally reaches the villain’s stronghold, the obligatory plot twist happens, with or without subsequent redemption of Hero1, with or without the reader being left to wonder whether a Hero3 would eventually be necessary.