duad vs couplet

duad and couplet both are nouns.

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

As nouns, duad and couplet are synonyms defined as:

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

duad (noun)	couplet (noun)
two items of the same kind	a stanza consisting of two successive lines of verse; usually rhymed
	two items of the same kind