I'm impressed! I've talked about press kits whenever someone asks about marketing. And, lo, here you are asking a great question.
In advertising, there are two major categories (greatly oversimplifying here): brand advertising and product advertising. Or, you might say author advertising and book advertising. I think, however, that with something like a press kit, you can push both almost equally.
One problem is that you have talking about two books, and different books at that. Even if they have different audiences, you are likely to be going to the same, or overlapping, media, interviewers, reviewers, etc. Personally, I'm of the the-more-the-better school. Two books are better than one. That is personally. When I was in advertising, I read Advertising Age regularly. There was almost always something about the McDonald's v. Burger King rivalry. What struck me about it was this: every time Burger King came out with a new sandwich it's market share dropped.
OK . . . so BK came out with a chicken sandwich and it's customer base, what, forgot they made burgers!? Down 1-3 points. Huh!? Honestly, I do not get it. But that pattern was consistent, not just for them but other products.
Arguably two different books are like two different sandwiches.
Honestly, I don't know. But that is what I've observed. Perhaps if you have different people/organization you approach for each book, or maybe stagger the contacts so what might be a follow-up 6 weeks or so later, could use the other book.
Food for thought, but I'm guessing you've already thought all this.
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