We don't have synonyms for doubletrees yet — it may be a rare, specialized, newly coined, or misspelled word. The closest thesaurus entries we have are doubletree, double reed and double-reed.
(n) :
a device that connects two horses to a wagon or other implement. The tugs of a harness are connected to a singletree, two of which are connected to a doubletree, which, in turn, is connected to the implement.
(n) :
a device that connects two horses to a wagon or other implement. The tugs of a harness are connected to a singletree, two of which are connected to a doubletree, which, in turn, is connected to the implement.