twain vs doubleton

twain and doubleton both are nouns.

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

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

twain: two items of the same kind
doubleton: (bridge) a pair of playing cards that are the only cards in their suit in the hand dealt to a player

twain (noun)	doubleton (noun)
two items of the same kind	(bridge) a pair of playing cards that are the only cards in their suit in the hand dealt to a player