Try this - Using iPhone, iPad, or iPod with multiple computers - http://support.apple.com/kb/HT1202.
I have 2 macs--a MacBook Pro and an iMac. I have ONE Apple ID (iTunes ID) that is registered on both machines. I used to sync my iPhone from the iMac, but now the iMac is in storage. I want to move some music from my MacBook to my iPhone and it wont let me...something about only being allowed to be synced to one iTunes library. It would erase everything on my phone if I wanted to add anything from my MacBook's iTunes library. Why isnt it the same iTunes library--it has the same iTunes ID/Apple ID? Is there a solution to this that isnt really annoying and redundant (like mirroring my two computers' library)?
This cannot be the first time someone has had this issue and Apple hasnt addressed it?
thanks Karen, this was helpful...but, when I follow the step to manage manually on my second computer (the MacBook) it says that this will erase my iPhone. Do I have to set the iPhone preferences to manage manually from my "home" computer? (if that is the case, I will have to wait since its in storage...)