duo vs twain

duo and twain both are nouns.

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

As nouns, duo and twain are synonyms defined as:

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

duo (noun)	twain (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