two vs duo

two and duo both are nouns.

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

As nouns, duo is a hyponym of two; that is, duo 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
duo: two items of the same kind

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

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

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