A good start might be to keep your first Qubes xserver :0, and to run a second one for your passed-through GPU.
The link in the comment by neowutran in the other thread contains a xorg.conf stub, which is adjusted by a script xorgx2.sh (I think).
The important bit AIUI is the BusID, which must point to your PT card, and the startx, which must use a new unused server id- usually :1 - and must use the xorg.conf file.
Then you can run apps on the PT GPU by prefixing the command like this:
DISPLAY=":1" glxgears
A similar method has worked for me in the past. Maybe it will be a starting point.