Geoffrey,

You can NOT stack walls. This gives unwanted side-effects.

1 - If you want to make a wall sloping, make it sloping.
2 - If you must have two different parts, make the sloping wall separately, use Tools --> Copy As New Furniture and then place it on top of the wall. Then, the wall isn't a wall anymore, but is converted to a normal furniture object, and those are stackable. This solution requires that you have the plug-in installed. (manual )
3 - If just the end-section of a longer wall should be sloping, select the wall, then do right-click --> Split walls. Adjust the length of the two sections, then make the end section sloping.

Hans