That About my Mac has nothing to do about "primary GPU", it just pick one and display. And since only one of your card has Mac EFI UGA, I suspect that GPU will registered first, therefore, always display that in About my Mac.
As long as any non-Metal GPU is installed. You can't run Mojave installer, regardless what shows in About my Mac.
There is no "primary GPU" in macOS. For compute, depends on software, you may able to assign a dedicated GPU for that (inside the software), or all will be used.
For 3D, the GPU that connected to the display is the only display that can do the job, not primary or secondary, etc.
For boot screen, only the Mac EFI UGA GPU can do that on cMP.