There was this vendor on Ragbrai with 3 or 4 of these side by side tandems. Two riders, 4 wheels. The front wheels are direct drive and fixed so there is no steering. The rear wheels are swivel casters. It requires the two riders to match their pedaling speed to go straight. A nearly impossible task. Pretty funny to watch them spinning around. The format is a race to a cone about 100 feet and return.