Thanks Gaieus, got it.
@gaieus said:
Hi Andy,
Create a 3rd layer for the top face of the ground floor walls. Now open that group, double click on the top face and copy it on the clipboard (Ctrl+C). While that top face+its bounding edges are still selected, right click and hide.
Now close the group and paste the copied top face in place (Edit menu). Make it a group, too, and set the visibility to be ONLY visible in Scene 1 (with groundfloor) while hidden in Scene 2 (with second floor, too).
Now open the second floor group, too, double click on its bottom face and hide that, too.
See attached file for ideas...