Comping my adult novel to YA novels

Is it okay to use comps outside one's category?

I'm currently working on an adult novel (historical fiction), but the two comps that feel right to me are both YA. Is it okay to say something like "I hope it will appeal to adult fans of [YA title] and [YA title]?" Or is this a red flag?

I read widely in the genre (both YA and adult), so I'm sure I can come up with some adult titles if this is an issue, but the two YA books feel like such a great fit (and, bonus, both are recently published and seem to have done well). It's probably also worth mentioning that there's no way to age my book down to YA (both because of the age of the protagonist and the subject matter).

I'm probably overthinking this, right?

You're not overthinking this at all.
This is a really good question, and I'm glad you asked.

Since you read both YA and adult you know that YA books differ from those marketed and sold as adult fiction even if they are both in the same genre (such as crime fiction, or historical fiction.)

YA generally is about young people finding their way in the world, making sense of situations they didn't create, or have thrust upon them because of their age.

Adult fiction can certainly be about that but the characters, as you point out, are older, have more life experience, and generally a few more of life's scuff marks.

YA is acquired by entirely different editors than acquire adult fiction (mostly), sold differently (mostly) and even in a separate imprint at the publisher (often.)

If I have a YA novel, regardless of genre, I go to the Harper divisions that publish those books.
If I have an adult novel, I go to the Harper imprints (like Morrow or Avon) that publish those kinds of books.

What you can intuit from that is that YA comps are largely meaningless to adult editors. Sure, adult editors have read some YA and they know what it is, but they're not steeped in it like the YA editors are.

The purpose of comps is to tell an agent or an editor what kind of reader will like your book. Given the size of the historical fiction market, you need to cast a wider net.

Don't feel stupid about this. Comps are the bane of my existence and I spend HOURS trying to get them right. And even then, when the brain trust here at New Leaf weighs in, someone else always has a better one (which is a good thing, but humbling!)

This may be a place where your librarian can help you. They know more (or at least how to find more) than any of the rest of us!