The Collaborative International Dictionary of English v.0.48:

Dos-a-dos \Dos`-[`a]-dos"\, n.
   A sofa, open carriage, or the like, so constructed that the
   occupants sit back to back.
   [Webster 1913 Suppl.]

The Collaborative International Dictionary of English v.0.48:

Dos-a-dos \Dos`-[`a]-dos"\, adv. [F.]
   Back to back; as, to sit dos-[`a]-dos in a dogcart; to dance
   dos-[`a]-dos, or so that two dancers move forward and pass
   back to back.
   [Webster 1913 Suppl.]