Can't stand that guy. I'll dangle my fat balls in his face, paint them to look like buckeyes first.
Technically, if they both start the game (i.e. they were both on the field for the first offensive play) they share the starting role. Realistically, if you dominate your carries and are on the field for the most important drives (first several series and again to strengthen the lead) you are the starter. Cheers.