@Bushmeister Frogger and Pac-Man (and many others) only had one set of controls; there was no need to install a second set of hardware, since the players take turns anyway. As far as the game is concerned, there aren't any "player 2" inputs except for the second coin slot and the 2-player start button (select and start on the second controller.)
You could remap this so that both sides control "player 1" (except for start button of course) and you can even make it only for these games so that 2p simultaneous still works on other titles -- the only catch is, they will both be always active so you have to trust the other person not to interfere when it's your turn.