The Unicyclopedia/Double wheeler
The double wheeler, also known as the biverticycle or two-wheeler unicycle is a vehicle with two wheels, where the wheels are stacked on top of each other. The top wheel is pedalled by direct drive. This drives the bottom wheel, which moves in the opposite direction.
By the same principle, multi-wheel unicycles have been built, with wheels often stacked in parallel, some with as many as 16 wheels.