dos-à-dos

from The Century Dictionary.

• noun Back to back; specifically, in dancing, an evolution in reels, etc., in which two persons advance, pass around each other back to back, and return to their places.

from the GNU version of the Collaborative International Dictionary of English.

• noun A sofa, open carriage, or the like, so constructed that the occupants sit back to back.
• adverb Back to back.