cosignatory vs signer

cosignatory and signer both are nouns.

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

As nouns, signer is a hypernym of cosignatory; that is, signer 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)
signer: someone who signs and is bound by a document

Other hypernyms of cosignatory include signatory.

cosignatory (noun)	signer (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
	someone who can use sign language to communicate

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