You'll need a Golden Key that you get from a Borderlands 2 pre-order code in order to do this.

Use a second controller to sign in a second player to your game. You will need to do this on a seperate account (a free one is fine).

Have the second player open the Gold Loot chest in the Sanctuary and then get player one (the one with the Golden Key) to loot it.

This will not use the Golden Key that the first player has!

You can do this as much as you want.