twain vs duo

twain and duo both are nouns.

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

As nouns, twain and duo are synonyms defined as:

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

twain (noun)	duo (noun)
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