From the creator of MyTube, MultiTunes is an app that allows you to sync with as many iTunes libraries as you want!
To get started, use the app to create a new library. Tap the library in the list to switch to it, then sync with another iTunes library to add content to your new library. You can add as many libraries as you want as long as there is enough space on your device. Then, use this app to switch between your libraries whenever you want.
IMPORTANT: Before downloading content from the iTunes or App Store apps, make sure your default library is selected in this app. Otherwise, your content may not download properly.