twosome vs twain

twosome and twain both are nouns.

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

As nouns, twosome and twain are synonyms defined as:

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

twosome (noun)	twain (noun)
a pair who associate with one another	two items of the same kind
two items of the same kind