two vs duet

two and duet both are nouns.

Word	Noun	Adjective	Verb	Adverb
two	Yes	Yes	No	No
duet	Yes	No	No	No

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

two: the cardinal number that is the sum of one and one or a numeral representing this number
duet: two items of the same kind

Other hyponyms of two include craps, snake eyes, brace, couple, couplet, distich, duad, duo, dyad, pair, span, twain, twosome, yoke.

two (noun)	duet (noun)
one of the four playing cards in a deck that have two spots	(ballet) a dance for two people (usually a ballerina and a danseur noble)
the cardinal number that is the sum of one and one or a numeral representing this number	a musical composition for two performers
	a pair who associate with one another
	two performers or singers who perform together
	two items of the same kind

two (adjective)	duet (adjective)
being one more than one