The Collaborative International Dictionary of English v.0.48:

Twin \Twin\, v. i. [imp. & p. p. Twinned; p. pr. & vb. n.
   Twinning.]
   1. To bring forth twins. --Tusser.
      [1913 Webster]

   2. To be born at the same birth. --Shak.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Twinning \Twin"ning\, n. (Crystallog.)
   The assemblage of two or more crystals, or parts of crystals,
   in reversed position with reference to each other in
   accordance with some definite law; also, rarely, in
   artificial twinning (accomplished for example by pressure),
   the process by which this reversal is brought about.
   [1913 Webster]

   Polysynthetic twinning, repeated twinning of crystal
      lamellae, as that of the triclinic feldspars.

   Repeated twinning, twinning of more than two crystals, or
      parts of crystals.

   Twinning axis, Twinning plane. See the Note under Twin,
      n.
      [1913 Webster]

WordNet (r) 3.0 (2006):

twinning
    adj 1: producing two offspring at a time [syn: biparous,
           twinning]

The Free On-line Dictionary of Computing (30 December 2018):

twinning

    Keeping a mirror of a magnetic tape.

   (1997-07-18)