couple vs conjoin

couple is a noun but conjoin is not a noun.

Word	Noun	Adjective	Verb	Adverb
couple	Yes	No	Yes	No
conjoin	No	No	Yes	No

As verbs, conjoin is a hypernym of couple; that is, conjoin is a word with a broader meaning than couple:

Other hypernyms of couple include join.

couple (noun)	conjoin (noun)
a pair who associate with one another
a pair of people who live together
(physics) something joined by two equal and opposite forces that act along parallel lines
two items of the same kind
a small indefinite number

couple (verb)	conjoin (verb)
engage in sexual intercourse	make contact or come together
link together	take in marriage; married
bring two objects, ideas, or people together
form a pair or pairs