A bicycle with two seats arranged one behind the other is called a tandem. The name describes the fore-and-aft seating arrangement, rather than simply indicating that two people ride it.
On a conventional tandem, the front rider—often called the captain or pilot—steers and pedals. The rear rider, known as the stoker, navigator or rear admiral, pedals while relying on the captain for steering. The two cranksets are usually linked by a timing chain, so the riders pedal in coordination.
Tandems have a long history: patents for them date from the mid-1880s, and the bicycle became a popular leisure machine for couples. Tandem racing even appeared at the Olympic Games from 1908 through 1972. “Twin” can be used informally, but it is not the usual quiz or technical term. A bicycle carrying riders side by side is instead called a sociable, while larger fore-and-aft machines may be called triplets, quads or quints.