duo vs duad

duo and duad both are nouns.

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

As nouns, duo and duad are synonyms defined as:

Other synonyms of duo include brace, couple, couplet, distich, duet, dyad, pair, span, twain, twosome, yoke.

duo (noun)	duad (noun)
a musical composition for two performers	two items of the same kind
a pair who associate with one another
two performers or singers who perform together
two items of the same kind