I’ve got the stock Nvidia GT 120 and an AMD RX 560 instated in my Mac Pro 4,1 (5,1 firmware) running High Sierra. I’ve got one screen plugged into the GT 120 and another plugged into the RX 560. Interestingly when I go to “About This Mac” the “Displays” tab shows both screen as connected to the...
If you don't have time to read my story (which also describes some interesting technical approaches) just scroll down this thread until a "100% WORKING SOLUTION" text
Discrete AMD GPU of my 2011 MacBook Pro 8,2 has finally failed because of the reasons mentioned here (...