twain vs duet

twain and duet both are nouns.

Word	Noun	Adjective	Verb	Adverb
twain	Yes	No	No	No
duet	Yes	No	No	No

As nouns, twain and duet are synonyms defined as:

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

twain (noun)	duet (noun)
two items of the same kind	(ballet) a dance for two people (usually a ballerina and a danseur noble)
	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