two vs duad

two and duad both are nouns.

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

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

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

two (noun)	duad (noun)
one of the four playing cards in a deck that have two spots	two items of the same kind
the cardinal number that is the sum of one and one or a numeral representing this number

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