Question : Problem: How do I add a second iPhone to iTunes

I have iiTunes set up to sync and back up my iPhone. My wife has a similar iPhone (both the latest 3G) and I want to be able to back her phone up on my computer. I cannot figure out how to add a second device.

Answer : Problem: How do I add a second iPhone to iTunes

It would be the same as using two ipods with the same computer.  The instructions (from Apple) are here:

http://support.apple.com/kb/HT1495