I'll only read two books at once if only one of those is a novel -- for instance, I'm currently reading China Miéville's Railsea, and I'm also making my way through the Warhammer Fantasy rulebook. The closest I come to reading two books at once is if I start a book, decide ~100 pages in that it's not what I'm in the mood for, and then pick up something different. I'll read that something different from cover to cover before going back to the original book.