I may be a little late for the party, but I hope I can still comment on this.
I wrote one novel with two
third-person characters (not narrators - I think that's confined to 1st-person), and it's not easy. If you enjoy planning a novel in advance (as I do), you have double the work than just one MC.
The best advice I've seen here is this:
Yeah, I think it’s critical for the voices to be distinct. The reader needs to be able to tell immediately whose perspective they’re in in a given chapter or scene.
X Eqestris is right on the money. If you have two distinct narrators, keeping their voices separate is crucial. Here's an idea how this can be done:
Think about your hero and your villain. Their backgrounds - education, upbringing, family - stuff like that. Maybe one of them was bullied at home? Maybe his father beat him while his mother protected him, so he grew up with attachment issues.
Or, maybe he didn't do well at school (for whatever reason), so he was bullied by the other kids - or the teacher? - and so he grew up with resentment towards his peers or towards authority figures, respectively. The result? Resentment towards peers could mean inability to function in a "working as a team" environment, preferring to be alone, etc. Resentment towards authority figures could mean suspicion towards bosses, outbursts at work ... maybe? Either way, bullying can seriously mess a person up.
All right, let's forget bullying. What your hero's/villain's hobbies? Let's say your villain is found of chess. Chess can imply two things: long-term, strategic thinking (the criminal mastermind type) ... or short-term, tactical strokes (brilliant but violent man of action). On the other side, your hero being a strategic thinker could make him a police chief or a general; being a tactical thinker could make him a PI. Either way, give them two or three hobbies to help define them.
Yes, I know I've been going on and on about this.

Like I said, I'm a planner. Plans can be tricky. But if you figure out your characters' backgrounds in advance, they can help you form a distinct voice.
One last thing: your characters are yours, and you probably know them much better than us. If you've thought this through and you know what your characters are like, please feel free to ignore everything I just said. I just hope this helps you anyway.
