The Collaborative International Dictionary of English v.0.48:

Two-ply \Two"-ply`\, a.
   1. Consisting of two thicknesses, as cloth; double.
      [1913 Webster]

   2. Woven double, as cloth or carpeting, by incorporating two
      sets of warp thread and two of weft.
      [1913 Webster]

WordNet (r) 3.0 (2006):

two-ply
    adj 1: having a thickness made up of two layers or strands