If you open device manager on your device, then look for display adapters, these should be ordered in the order of their index (i.e. the top one is zero, the second one is one etc…)
So at the bottom of the config file ConsoleVariables.ini, on a new line add r.GraphicsAdapter=x
And replace X with the index of the display adapter that you want UE4 to use. If not sure trial and error is normally a good option, just move through the list until it works as intended.
I’m no expert myself but this helped me understand the problem, although the problem with my system ended up being much larger than just this. I believe the fix mentioned here should completely solve the problem for most people.