Patch 22 includes a new feature which is rubber band co-op:
Open the friends menu and "Start rubber band pairing". A rubber band will spawn between you and any other nearby player who's in rubber band pairing mode as well.
If you manage to beat the game from start to finish in one sitting while being tethered to at least one other player you will unlock sunglasses (hat category) and a bathing summer outfit:
Players on patch 22 and onward will be put in different lobbies than those running 21 and prior.
Thank you Bobandal for the idea and extensive beta testing!