tandem (n.)

1785, "carriage pulled by horses harnessed one behind the other" (instead of side-by-side), jocular use of Latin tandem "at length (of time), at last, so much," from tam "so" (from PIE *tam-, adverbial form of demonstrative pronoun root *-to-; see -th (1)) + demonstrative suffix -dem. "Probably first in university use" [Century Dictionary]. Transferred by 1884 to bicycles with two seats. In English as an adverb from 1795; as an adjective from 1801.

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Definitions of tandem
tandem (n.)
an arrangement of two or more objects or persons one behind another;
tandem (n.)
a bicycle with two sets of pedals and two seats;
Synonyms: bicycle-built-for-two / tandem bicycle
tandem (adv.)
one behind the other;
ride tandem on a bicycle built for two
riding horses down the path in tandem
Synonyms: in tandem