macfrog,

Of course there is a solution.
1 - Download this arch from the 3D models section.
2 - Import it twice: first time normal, second time rotate it until upside-down. Both times make sure to check the box Door/Window.
3 - Insert both arches in your design, give them the same coordinates, then give the upper one the correct elevation, and group them.
4 - Resize the grouped arches to the correct size and elevation, then insert them in your wall.

Hans