I already have all my iPhone content backed up on my iMac. As recently my wife got a new iPhone for herself, I would like to know how to back up her phone content using the same computer (I guess it's possible)
iMac
iTunes is intelligent enough to discriminate between the two. They won't become confused with one another.
Unless you want them both to have the same content though, you might want to consider creating separate User Accounts. That way, each one can be synced to its own iTunes Library, have different apps, different Contacts, etc.
iTunes: How to share music between different accounts on a single computer
How to use multiple iPods, iPads, or iPhones with one computer
How can we back up two iPhones on the same iMac?
