co-sign vs validate

co-sign and validate both are verbs.

Word	Noun	Adjective	Verb	Adverb
co-sign	No	No	Yes	No
validate	No	No	Yes	No

As verbs, validate is a hypernym of co-sign; that is, validate is a word with a broader meaning than co-sign:

Other hypernyms of co-sign include formalise, formalize.

co-sign (verb)	validate (verb)
sign and endorse (another person's signature), as for a loan	make valid or confirm the validity of
sign jointly	prove valid; show or confirm the validity of something
	give evidence for
	declare or make legally valid