twain vs twosome

twain and twosome both are nouns.

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

As nouns, twain and twosome are synonyms defined as:

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

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