First you need a thumb drive or a Playstation plus membership. Back up a save file to either one.
1. Load a two player game with the second player having a golden key (and being the save file backed-up to either thumb drive or Playstation plus cloud.)
2.Open chest with second player, take the chest contents and give it to player one.
3.Save and quit game.
5.Copy backed up save file to hard drive.
Player one save file will have chest contents and player two will have the key back. Repeat as many times as desired.