That's not at all uncommon. I've jokingly suggested the idea that authors band together in a co-op so we can all promote each others' books without having to hawk our own. The trick is that everyone in the group has to feel the same way because it won't work if everyone agrees to that, and then there are one or two people who let everyone else promote their books, and then they don't promote the others (which has happened when I've been in groups that tried this). And you have to genuinely like the other people's work. It's awkward when you're obligated to push someone else because of an agreement where you promote each other, but then you're not so crazy about that one book.

I used to work in PR. I can do great PR for other things I believe in. Me, not so much. I feel icky.