The Collaborative International Dictionary of English v.0.48:

Twain \Twain\ (tw[=a]n), a. & n. [OE. twein, tweien, tweyne, AS.
   tw[=e]gen, masc. See Two.]
   Two; -- nearly obsolete in common discourse, but used in
   poetry and burlesque. "Children twain." --Chaucer.
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         And whosoever shall compel thee to go a mile, go with
         him twain.                               --Matt. v. 41.
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   In twain, in halves; into two parts; asunder.
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            When old winter split the rocks in twain. --Dryden.
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   Twain cloud. (Meteor.) Same as Cumulo-stratus.
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