I once came across a code which allowed the Aztec level to be played in multiplayer. It didn't have any weapons of course, but that could be fixed in a way using the All Guns cheat.
Try this: No weapons, one-hit-kills. Go to the room with the pit in the middle, which is the second room from where you start in single player. Have one player stand on one side of the pit, facing it, and the other player on the opposite side. Count to three, and then run straight at each other, into the pit, and try to hit each other whilst jumping. A true duel!