1.) There are two necklaces that can be worn by two people. Once both parties have the necklaces on, they are given the ability to swap whatever they like with each other. If only one person has the necklace on, the ability doesn’t work.
2.) There is a single ring that gives the user the ability to pick and choose who and what they want to swap with others. When the ring is removed, the user cannot swap anything and instead is stuck with whatever combination of parts and or traits they’ve swapped until they put the ring back on.