My wife and daughter each have an iPhone, and I find that using iTunes is kind of a pain. I used to sync up my daughter’s iPhone, but now she claims that she loses a whole bunch of data (phone numbers etc.) whenever it gets synced, so now won’t let me sync her iPhone to the computer, thus failing to get whatever new songs and apps are loaded in it. Maybe I was somehow doing it wrong, but if so, it’s not very intuitive.
The best way to handle iPhones is to have each synch to a separate user account on the computer. That way each will have it's own address book, photos, and playlists in iTunes. You can share an iTunes store account so apps and tunes can be purchased only once and shared across the devices.