[papalalu]

a sticker (i suppose peelable ideally) on the back of each other's device could give some interesting options..

i also think they are missing a trick not having the pattern on the front and using the then known position of the wearer's head in some way..

perhaps the two could be combined into some kind of shield/shooter duel, in which case you could still flog the tshirts as part of the game.