COSIGNATORY

cosignatory

(adjective) signing jointly with others

cosigner, cosignatory

(noun) one of two or more signers of the same document (as a treaty or declaration)

Source: WordNet® 3.1

Etymology

Adjective

cosignatory (not comparable)

signed or signing jointly

Noun

cosignatory (plural cosignatories)

Any of several people who sign a document together (especially a treaty)

Source: Wiktionary