dos a dos

Etymology
From, from (from ) and  (from.

Noun

 * 1) A move in square dancing in which two dancers face each other, then step forward and left until they have right shoulders adjacent, then move to a position where they are back to back, then move to have left shoulders adjacent, then return to facing each other.
 * 2)  A sofa, open carriage, etc. constructed so that the occupants sit back to back.

Adverb

 * 1) Back to back.

Adverb

 * 1) mutually