sleep together vs pair

sleep together is not a noun while pair is a noun.

Word	Noun	Adjective	Verb	Adverb
sleep together	No	No	Yes	No
pair	Yes	No	Yes	No

As verbs, pair is a hypernym of sleep together; that is, pair is a word with a broader meaning than sleep together:

Other hypernyms of sleep together include copulate, couple, mate.

sleep together (noun)	pair (noun)
	two people considered as a unit
	a set of two similar things considered as a unit
	a poker hand with 2 cards of the same value
	two items of the same kind

sleep together (verb)	pair (verb)
have sexual intercourse with	engage in sexual intercourse
	bring two objects, ideas, or people together
	arrange in pairs
	occur in pairs
	form a pair or pairs