Alternating POVs--teenaged protagonist and adult villain?

An agent would know better, but I suspect this would be a hard sell unless the adult POV appears significantly less than the teen POV. If it’s a sixth or a tenth of the book, say. If it’s half the book, and the adult villain is a twenty something ... maybe? If the villain is older and it’s 50/50, it might need to be an adult book.

I kind of got away with adult POV by giving my narrator a psychic link to a twenty-something character, so there are special cases. His POV is super-dark, too. But it’s not a huge part of the book.

My understanding is that YA should be told entirely or primarily from the pov of a teen or teens.

A story that shares pov equally between the protagonist and antagonist seems like it would actually be just as much about the antagonist. Or maybe (regardless of character ages) such a book wouldn't be as much about the protagonist's journey as it would be about the conflict between these two people?

There are, as the previous poster mentioned, special exceptions to the rules for almost everything, though. I'd definitely seek opinions from critting partners and beta readers who understand the genre and can give specific feedback on this matter.

I don't think dual protagonists itself is an issue; my current WIP is dual POV, but falls firmly into YA due to both POV characters being teens, even though one of them is very much a villain. I think once you make one of the POVs an adult you leave the YA world, as YA always has teens as their POV characters. That doesn't mean your book can't work, you should just maybe consider a label other than YA.

I think it might also depend on the genre and tone of the book. How dark is the villain? Fine art thief? Serial killer? Kidnapper? What are the stakes? What exactly is their relationship like? Do they know each other at the beginning of the book? How do they know each other? Is it equal POV right from the beginning?

With dual POV, if both characters are represented equally I always feel like it is "both" of their books. But some books with dual POV still lean more towards one character - so in your case, does the book lean more towards the teen character?

And of course there are exceptions to every rule. The Book Thief mc is around 12 but was first sold as adult fiction and then YA. The book Room is told entirely from the little boy's pov, but is definitely adult fiction. There are many more.