Multiple itunes sync

If trying to sync or manually manage iTunes content with more than one computer, that's exactly what will happen, all iTunes content will first be erased from your phone. This is a design feature of the iPhone, it will sync or manually manage iTunes content with one computer at a time.

Miss this part of that article, at the bottom?

iPod shuffle and iPhone are intended for use with a single computer. You cannot load music from multiple computers or iTunes libraries onto iPod shuffle and iPhone like you can with other devices.