Need help with loading music

I have a 2gen ipod touch and I just got the iphone4. My ipod touch is synced with my imac and (for work needs) i want iphone4 to sync with my Macbook Pro.
On the ipod touch I can add music from either machine. I have the box "manually manage music and video" checked.
On the iphone4 I check that same box on the macbook pro and I can add music no problem. if i plug it into the imac that box is no longer checked and i cant add any songs to the iphone.
Does anyone know why this is? I appreciate all help,

Oh ipod and iphone are running iOS4 and both computers are running itunes 9.2


Chris