'Doubletree' definitions:

Definition of 'doubletree'

(from WordNet)
noun
A crossbar on a wagon or carriage to which two whiffletrees are attached in order to harness two horses abreast

Definition of 'Doubletree'

From: GCIDE
  • Doubletree \Dou"ble*tree`\, n. The bar, or crosspiece, of a carriage, to which the singletrees are attached. [1913 Webster]