duet vs twain

duet and twain both are nouns.

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

As nouns, duet and twain are synonyms defined as:

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

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