You can double team a WR. In Madden you can assign certain players in your back 7 to certain responsibilities, but for some stupid reason you can't in NCAA, which creates huge problems when people put their best reciever in the slot going against your nickleback.
You can't double team a DL, but it doesn't really matter. The line interaction is completely random anyway. A 94 overall DT could get pancaked 10 times in a game by a 75 overall gaurd, while your All American 99 overall OT gives up 3 sacks to an 81 overall DE. Doubleteaming would probably create more problems than it would solve.