Whether you have an iPod, an iPhone, or an iPad, you probably sync your music and video library to your iDevice using iTunes. That’s fine for your own collection, but what if you have more than one iTunes library? That’s where you might want to give MultiTunes a try.
You will need to jailbreak your iDevice first, since MultiTunes is only available (for free!) through the Cydia Store, but it effectively lets you sync to multiple iTunes libraries on a single device. The libraries don’t really merge though. Instead, you have to switch between them on the fly if you want to access the different sets of tunes.
First, you select the library in MultiTunes and then you sync the corresponding library with the computer. You repeat the process with new libraries. When you want to listen to your music, you go into the MultiTunes apps and pick the library you want, flipping over to the Music or iPod app to find the tunes in that library. When you want music from a different library, you go into MultiTunes to pick that one instead before going back to the Music or iPod app.[ source via Harrison Apps ]