a bicycle for two riders, one behind the other 双座自行车；双人自行车 
Word Originlate 18th cent.: humorously from Latin, literally ‘at length’.
Idioms
in tandem (with somebody/something)
if somebody/something works or happens in tandem with somebody/something else, they work together or happen at the same time 并行；并驾齐驱；同时实行 The two systems are designed to work in tandem. 这两套系统是为同步工作而设计的。 Two deisgners worked in tandem on the plans. 两个设计者协同工作制定计划。 Stock prices pushed sharply higher this afternoon in tandem with a rise in the dollar. 随着美元的升值，今天下午股票价格急剧上涨。