The Collaborative International Dictionary of English v.0.48:

Cosignitary \Co*sig"ni*ta*ry\ (k?-s?g"n?-t?-r?), a. [Pref. co- +
   sign. Cf. Signatory.]
   Signing some important public document with another or with
   others; as, a treaty violated by one of the cosignitary
   powers.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Cosignitary \Co*sig"ni*ta*ry\, n.; pl. Cosignitaries (-r?z).
   One who signs a treaty or public document along with others
   or another; as, the cosignitaries of the treaty of Berlin.
   [1913 Webster]