The rules would be that you can grab, give power ups, or do anything with the second player, but you can't touch their controller whatsoever. You can also them as a checkpoint if you know you will die from getting a star coin if you do a 100% run. As long as you don't touch the player 2 controller, your run will be okay.

This roun would be really interesting to see since there would be some creative barriers along the way.