How come there are two number 10's on USC's football team?
Brian Cushing and David Booty.

Yea, I understand the "No two on the field with the same number." I just thought that the starting quarterback would have his very own jersey number.

Pretty common in college football. Unlike the pros, there's no limit to how many players can be on a team (although there is a limit on how many can travel). Nebraska used to have well over 100 players every year.

Obviously, with that many players, you run out of jersey numbers after 100 (don't forget 00). So, you wind up with a few sets of duplicates. And it's fine. You just can't have two of the same number on the field at the same time.

happens alot with teams with alot of retired numbers.. right now on Auburn #18 is Wes Byrum the FG kicker and Kodi Burns the back up qb.. also #1 is the punter and Brad Lester.. they just run out of numbers but the rule is that both players with the same number cannot be on the field at the same time

They just can't be on the field at the same time, numerous amounts have dual number jerseys.