The use of the term "ply" is misleading.....
All commercial aircraft I have worked on have a window assembly comprised of a outer primary pane and a thinner inner (back-up) pane which both sit in the seal.
For Boeings the windows are on-condition and are usually either replaced for poor condition (scratches etc) or misting between the panes. Usually indicates a leaking seal or deformation of the inner pane.
Hope this helps