cosignatory vs cosigner

cosignatory and cosigner both are nouns.

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

As nouns, cosignatory and cosigner are synonyms defined as:

cosignatory and cosigner: one of two or more signers of the same document (as a treaty or declaration)

cosignatory (noun)	cosigner (noun)
one of two or more signers of the same document (as a treaty or declaration)	one of two or more signers of the same document (as a treaty or declaration)
	a signer in addition to the principal signer (to verify the authenticity of the principal signature or to provide surety)

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