partner vs copartner

partner and copartner both are nouns.

Word	Noun	Adjective	Verb	Adverb
partner	Yes	No	Yes	No
copartner	Yes	No	No	No

As nouns, copartner is a hyponym of partner; that is, copartner is a word with a more specific, narrower meaning than partner:

Other hyponyms of partner include silent partner, sleeping partner.

partner (noun)	copartner (noun)
an associate in an activity or endeavor or sphere of common interest	a joint partner (as in a business enterprise)
a person's partner in marriage
a person who is a member of a partnership

partner (verb)	copartner (verb)
provide with a partner
act as a partner