1 House, 2 iPhones, 1 iMac

So, there's probably an easy answer to this, but I wonder if anyone lives this way, personally:

My mom and dad are buying iPhones. They will each want to sync/backup their iPhones to the same computer--a new iMac. How will iTunes know to open a distinct folder/account for each of them, each time each iPhone is connected? Obviously we are worried that iTunes will accidentally sync the wrong iTunes account to the wrong phone and end up wiping out the contacts, etc. from that phone and over-riding it with stuff from the other iPhone account. See what I mean?

What steps do I need to take to prevent this from possibly happening (and, no, buying another computer is not an option right now. )