As others have said, neither is good English.

That being said, you're referring to two people, so are is the less incorrect version in that specific example. I'm way too long out of grammar classes to remember the right terminology for the underlying "rule", but it comes down to what the target is.

They are Trevor is a more compact version of they are both Trevor, which is again the short version of they are both named Trevor.

So, it comes down to sticking as close to that as possible. Their names are Trevor, because there's a single name, but plural people. Both people possess the name Trevor. Trevor is their name.

That's usually the way to figure out the least wrong/closest correct way to say something. You shift the wording until you get something that scans right to your brain, and most of the time you're as close as you'll get with conjugation of "to be"

And it's weird as fuck that "to be" is so transformed. Is, am, was, were, are, been, being, and even weird shit like "be going to" and "going to be". Like, wtf English? Why you gotta bring extra words to the party like that? You really need a second verb to conjugate a verb?

There's some weird etymology shit going on there somewhere between old English and it's roots. Hell, the fact that "to be" is even necessary is a little weird. Like "ser" in Spanish. It means the same thing, but lacks the weirdness of to as a butt buddy. Mind you, that shit conjugates weird too! Yo soy muy mindfucked by Spanish too, and I bet tu eres mindfucked tambien. Like, really, go look up the conjugation of ser. It isn't as wonky as to be, but it's getting there, especially in the imperfect tenses.

I don't even speak Spanish worth a damn, much less any other languages, but I bet every language has some fuckery like that in it. I'm just really gobsmacked at the verbal gymnastics of the two I'm familiar with. But the English version of the underlying idea is just an extra layer of tomfoolery.