Aloha mashinhead,
I have a slightly different setup, for a slight different need. I have my iTunes library on an external HDD, and point my iTunes on both my iMac and my MacBook Pro to that iTunes library. That way, when I go on vacation, I can take my MacBook Pro and the external HDD and all is well. With respect to my iPhone, as my iTunes library resides on my external HDD, it does not matter which computer I use to access iTunes. I have yet to attempt accessing iTunes concurrently, however.
One thing I like about this setup is that I don't have to set up two different "sync profiles" (for lack of a better phrase), as both computers are using the same iTunes library file.
EDIT: I just noticed that you referenced having the same user information on both computers. I have Standard User accounts on both computers with different usernames and passwords, but everything still works perfectly.
?HawaiiMacAddict