The Collaborative International Dictionary of English v.0.48:

Syngraph \Syn"graph\, n. [L. syngrapha, Gr. ?; sy`n with + ? to
   write.] (Law)
   A writing signed by both or all the parties to a contract or
   bond.
   [1913 Webster]

Bouvier's Law Dictionary, Revised 6th Ed (1856):

SYNGRAPH. A deed, bond, or other instrument of writing, under the band and
seal of all the parties. It was so called because the parties wrote
together.
     2. Formerly such writings were attested by the subscription and crosses
of the witnesses; afterwards, to prevent frauds and concealments, they made
deeds of mutual covenant in a script and rescript, or in a part and
counterpart, and in the middle between the two copies they wrote the word
syngraphus in large letters, which being cut through the parchment, and one
being delivered to each party, on being afterwards put together, proved
their authenticity.
     3. Deeds thus made were denominates syngraphs by the canonists, and by
the common lawyers chirographs. (q.v.) 2 Blackstone's Commentaries, 296.