Tandem Bicycles require that you have a partner or more. There are tandems on which more than two people can ride.
In the last years the interest in tandem bikes has increased strongly. Apparently it makes more fun to pedal along with a partner. Especially if you are sitting on the same bicycle with your girl-friend or boy-friend this is an amusing and exceptional experience.
When you are just two on one bike, talking is possible while driving. But if there are sitting three or more on one bike and your are steering it you better keep your attention on the road.
Just watch this funny video.
With two drivers pedaling you reach higher speed levels on even ground compared to a normal bicycle and pedaling in general requires less effort. If a driver is tired he can relax for a while and let the other pedal by himself.