First, start with a character that has the item, save it, then exit to menu.

Next, go in and hit [Start New Game] and hit two player, then the dificulty you were on (example: Champion) then import your character twice drop the items, and save over your old slot. since progress is tied to your character, all of your progress will be saved and all the Planes will still be open. keep repeating this process to get as many items you'd like.