II vs twain

II and twain both are nouns.

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

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

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

II (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