twain vs couplet

twain and couplet both are nouns.

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

As nouns, twain and couplet are synonyms defined as:

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

twain (noun)	couplet (noun)
two items of the same kind	a stanza consisting of two successive lines of verse; usually rhymed
	two items of the same kind