2 vs duo

2 and duo both are nouns.

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

As nouns, duo is a hyponym of 2; that is, duo is a word with a more specific, narrower meaning than 2:

2: 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 2 include craps, snake eyes, brace, couple, couplet, distich, duad, duet, dyad, pair, span, twain, twosome, yoke.

2 (noun)	duo (noun)
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

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