Usually but not all the time. Some books I might reserve for a particular location, for example in the car and indoors, others for a particular time, like one book to read during the day and another to be read right before going to sleep. Some of us like to totally immerse ourselves in the worlds of the characters in the book, even thinking about the story when not reading. For these people, one book at a time is the way to go. For others, I say go for it - don't worry about getting muddled up with the books. Our brains can do a lot, you know.

Maybe choose books that are of a different genre, format, or author.