couplet vs twain

couplet and twain both are nouns.

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

As nouns, couplet and twain are synonyms defined as:

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

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