Here’s what I wrote earlier:
There was an error which was fixed with the latest version of Nuendo. Is what I described not working for you?
Perhaps I should ask why you are splitting it into a top layer and a bottom one. Is it because you don’t have a 3D verb?
Anyway, I would think that if you create a quad group and you route it into a 7.1.4 group/bus you should be able to elevate it, no? And then front / rear should follow?