Tandem

Part of speech: noun (n.)

A team of horses harnessed one before the other. "He drove tandems." Thackeray. Tandem engine, a compound steam engine having two or more steam cylinders in the same axis, close to one another. -- Tandem bicycle or tricycle, one for two persons in which one rider sits before the other.

Part of speech: adverb (adv.)

One after another; -- said especially of horses harnessed and driven one before another, instead of abreast.