cosignatory vs signatory

cosignatory and signatory both are nouns.

Word	Noun	Adjective	Verb	Adverb
cosignatory	Yes	Yes	No	No
signatory	Yes	No	No	No

As nouns, signatory is a hypernym of cosignatory; that is, signatory is a word with a broader meaning than cosignatory:

cosignatory: one of two or more signers of the same document (as a treaty or declaration)
signatory: someone who signs and is bound by a document

Other hypernyms of cosignatory include signer.

cosignatory (noun)	signatory (noun)
one of two or more signers of the same document (as a treaty or declaration)	someone who signs and is bound by a document

cosignatory (adjective)	signatory (adjective)
signing jointly with others