Depending on the location of the window above the other one, you may apply the following tip:
In a new document, add a first window (or a door), a second window and eventually a separating box, and take care to put them on each other correctly (see the PorteBoiteFenetre.sh3d joined to the message).
Then export these items with the menu item 3D View > Export at OBJ format..., and import the generated OBJ file with the menu item Furniture > Import furniture..., checking Door or window option.