2 vs twain

2 and twain both are nouns.

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

As nouns, twain is a hyponym of 2; that is, twain is a word with a more specific, narrower meaning than 2:

2: 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 2 include craps, snake eyes, brace, couple, couplet, distich, duad, duet, duo, dyad, pair, span, twosome, yoke.

2 (noun)	twain (noun)
the cardinal number that is the sum of one and one or a numeral representing this number	two items of the same kind

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