二人三脚
ni.nin.san.kyaku

Literally: two – person – three – leg

Alternately: A “three-legged race”: an event (often seen in Japanese school Sports Days) in which competitors attempt to race in teams of two, with the right leg of one member tied to the left leg of the other. By extension, this phrase can refer to any situation with two people acting in concert or cooperating and coordinating to mutual benefit.

Notes: Phrases such as 一致協力 and  一致団結 are considered synonyms, although they refer to larger groups where this one is specifically about a partnership of two.

Image courtesy of irasutoya.com