Jon:
You can set the wall to
Cut Only, then adjust the
Floor Plan Cut Plane for each
View so the Cut Plane cuts through the lower window. Walls left on
Symbolic Cut will not be affected.
You should add a Signature to your Profile (click the Profile button near the top of this page) with your ArchiCAD version and operating system (see mine for an example) for more accurate help in this forum.
David