Press Z to activate the Weapon Menu
Hold X and Left-Shift
Release them and, bingo, a second player will be added!

The second player's life bar is your life bar (aka when he gets hurt, you get hurt). Also, the second player cannot use magic and has to use the same weapons as player 1. Also, levels don't seem to go up as normal while the second player is activated. He can be used to your advantage, however, try using him as a platform . . .

Thanks to Revolution reader Arrggghhh!