a crossbar on a wagon or carriage to which two whiffletrees are attached in order to harness two horses abreast
"The old farmer inspected the doubletree on his wagon before hitching up the pair of draft horses for the morning trip."
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.
"The old farmer carefully attached the doubletree to the plow before hitching up his team for morning work."