I didn't used to, but when I had to for college and grad school, it became second nature.
Usually, it's just a novel and non-fiction - I tend to read the non-fiction in chapters or sections, and read a novel all the way through before I start another.
However, I often have library ebooks with a deadline going, and I'll pause on what I"m reading elsewhere to finish quickly. Also, when I started reading on my phone and had an app for every thing, I ended up with different books going at the same time, novel or nonfiction. And now I've got a few eInk devices and the iPad. So it's not unusual to have a different book going in different apps or devices.
I took a look, and right now I have:
Kobo app on iPad - Doctor Who: A Writer's Tale
Kobo app on iPhone - nothing - still have to register new phone
Kindle app iPad - nothing - still organizing collections so I can import them to PW2
Kindle app iPhone - haven't registered app yet
Kindle PW2 - Eating Mindfully
Sony T2 - The Power of Rest and Eating Mindfully (switched to reading it on PW2 for now)
Sony T1 - nothing - too busy trying out the PW2
Nook app on iPhone - nothing yet - haven't registered the app yet on new phone
Nook app on iPad - If the Shoe Fits
iBooks on iPad - On a Wicked Dawn (a re-read)
iBooks on iPhone - finished a novel on Thursday
OverDrive app iPhone - just finished a novella last night
OverDrive app iPad - The Price of Everything (barely started)
3M library iPad - nothing right now - let a Coco Chanel biography expire because it was slow going
3M library iPhone - need to download to new iPhone when one of my holds comes up.
Yes, I'm fully aware that is just crazy. 6 books at once, and there 8 until I finished two.
I need to wrap some more of these up before more of library holds come up.