The Collaborative International Dictionary of English v.0.48:

Interpenetrate \In`ter*pen"e*trate\, v. i.
   To penetrate each the other; to penetrate between bodies or
   their parts.
   [1913 Webster]

   Interpenetrating molding (Arch.), in late Gothic
      architecture, a decoration by means of moldings which seem
      to pass through solid uprights, transoms, or other
      members; often, two sets of architectural members
      penetrating one another, in appearance, as if both had
      been plastic when they were put together.
      [1913 Webster]