formalise vs co-sign

formalise and co-sign both are verbs.

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

As verbs, co-sign is a hyponym of formalise; that is, co-sign is a word with a more specific, narrower meaning than formalise:

Other hyponyms of formalise include ratify, sign, cosign, probate.

formalise (verb)	co-sign (verb)
make formal or official	sign and endorse (another person's signature), as for a loan
declare or make legally valid	sign jointly