I never used to be able to read more than one book at once and if I ever did I often felt guilty doing so. It was as if I wasn't giving enough attention to the books that I was reading and felt that I should only be reading one at once. However, that has all changed now.
I often have a tree book and an e-book version on the go at the same time. The tree books I tend to read during the day and the e-book versions tend to get read at night on the laptop as I don't need a light on and don't tend to disturb the hubby when I'm reading quite as much as I did when I had a light on.
How often do you notice in the books that you're reading whether you read one at once or more than one at once that the stories might have some coincidences or similarities in them. For instance I'm currently reading two books that both have birds in them. The tree book A Kestrel for a Knave by Barry Hines features a Kestrel Hawk and the e-book Bellman & Black by Diane Setterfield features a rook.
It's odd when this happens, I've been known to read stories that have characters with the same first name in them one after the other or even places that they are set in. I don't consciously seek out these books it just happens.