The Collaborative International Dictionary of English v.0.48:

Subduple \Sub"du*ple\, a. (Math.)
   Indicating one part of two; in the ratio of one to two.
   [1913 Webster]

   Subduple ratio, the ratio of 1 to 2: thus, 3:6 is a
      subduple ratio, as 6:3 is a duple ratio.
      [1913 Webster]