I like the idea of doing a joint book launch with another author. I guess the only problem may be possible Q&A spoilers for readers who haven't read the other author.
I think you could specify no spoilers when you started the Q&A.
I thought the dual aspect worked really well, with a good dynamic, but the staff at that branch said it was the first they'd done. I guess it's not that common that you get two authors who know each other, have ties to the locality, are writing in the same genre, and each have a book out in the same month or so.
It must be more common at conventions, I'd have thought, where locality isn't important at all, and publication date less so.