Q: why am i not allowed to have more than one iTunes library associated with my iPhone?

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Nik,

The reason is that your iTunes Library and your iPhone are syncing devises, i.e., what is on your phone can be synced to your computer and vice versa. It would be impossible to coordinate two libraries as they would not know what one has done versus the other. You can actually transfer music from one library to another one rather easily and then sync from that one to your phone thus getting all your music. Just do a search about "transferring your iTunes library to another one" Also here is an Apple article that may help you.

How to move your iTunes library to a new computer - Apple Support

I hope this helped you....