twain vs couple

twain and couple both are nouns.

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

As nouns, twain and couple are synonyms defined as:

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

twain (noun)	couple (noun)
two items of the same kind	a pair who associate with one another
	a pair of people who live together
	(physics) something joined by two equal and opposite forces that act along parallel lines
	two items of the same kind
	a small indefinite number

twain (verb)	couple (verb)
	engage in sexual intercourse
	link together
	bring two objects, ideas, or people together
	form a pair or pairs