I did this yesterday and though it was pretty cool, so I figured I would share it

Requirements:

Borderlands 2 (duh)
2 Controllers
2 accounts
A saved game on each account.


Instructions:
1)Start up a split screen game

2)Player 1 should have the desired weapon in their backpack or equipped (dosent matter which)

3)Have player 2 join the game

4)drop the weapon you want duped and have player 2 pick it up

5)Leave the game as player 2 (press start and the B button)

6)After it saves the game for player 2 dashboard as player 1

7)Start up split screen game with the same profiles and you will see that both players have the weapon!

Note: This can be done on PS3 I assume but don't know because I have never played a PS3