two vs twain

two and twain both are nouns.

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

As nouns, twain is a hyponym of two; that is, twain is a word with a more specific, narrower meaning than two:

two: the cardinal number that is the sum of one and one or a numeral representing this number
twain: two items of the same kind

Other hyponyms of two include craps, snake eyes, brace, couple, couplet, distich, duad, duet, duo, dyad, pair, span, twosome, yoke.

two (noun)	twain (noun)
one of the four playing cards in a deck that have two spots	two items of the same kind
the cardinal number that is the sum of one and one or a numeral representing this number

two (adjective)	twain (adjective)
being one more than one