Just like the Bouncing Betty/perk cheat; in a 2 (or more) player game, have one player down themselves, have the second player drop his/her last Bouncing Betty (while the screen says to revive player hold X - or square) and immediately revive the downed player.

When you are done, your hands should be empty. Buy a gun for your new gun slot.

Use this early, before you do the perk cheats to gain an extra gun or two.