Um... CashCrazed, didn't you read what he said? In Resident Evil 0, you can control both characters at the same time - partner zapping as he called it. So it doesn't only work in puzzle games, huh?
By the way, I like that idea. Though you probably wouldn't be able to control both Mario and Luigi at the same time, you could switch back and forth between the characters... Or simply have the traditional 2-player mode, where the 2 players would take turns controlling Mario and Luigi. Or they could be both controlled at the same time by 2 players! Having Luigi fight an enemy while Mario runs ahead to get an item would be cool!