Source : Webster's Revised Unabridged Dictionary (1913)

Doubletree \Dou"ble*tree`\, n.
   The bar, or crosspiece, of a carriage, to which the
   singletrees are attached.

Source : WordNet®

doubletree
     n : a crossbar on a wagon or carriage to which two whiffletrees
         are attached in order to harness two horses abreast