Regarding this problem, I thought I'd try side by side comparision running p3d and fsx at same time - you can't do it.
If running p3d and then try to run fsx, it's says already running and same thing happens if you run fsx then run p3d, it says p3d is already running.
May the Mynd be with you.
So far my number of landings either passenger or pilot equal my number of takeoffs.
FSX Gold Acc, P3D V4, FS2020