tandem

Concise Oxford English Dictionary © 2008 Oxford University Press:
tandem/ˈtandəm/

▶noun

• 1 a bicycle with seats and pedals for two riders, one behind the other.

• 2 a carriage driven by two animals harnessed one in front of the other.

• 3 a group of two people or machines working together.

▶adverb one behind another.

– phrases
in tandem

• 1 alongside each other; together.

• 2 one behind another.

– origin C18: humorously from L., lit. ‘at length’.