I've been able to play music from two accounts at once. You're only logged into the STORE with one account at a time, for buying, but you don't need to be logged into the store at all (or even online for that matter) to be authorized to PLAY music. (You were online when you first authorized, but no need after that.)

I just did it a month ago to help my parents with something, and I'd swear their purchase AND my own library both played and synched to my phone OK.

I don't think most people NEED more than one iTunes account at once in a single user account. But if you do it does complicate things I'm sure.

Glad to see DRM is gone from iTunes music in April. (And almost gone already.)

Now, movies and audiobooks are a different matter.

STROGG: Change We Can Believe In