Bump
I'm having this problem also. But for a 1070.
Dual monitors previously worked on 10.13.2, but when I updated 10.13.6, it messed up.
The OS recognizes my second monitor, but the second monitor is just backlit. If I boot my Hackintosh and then login, my mouse is able to show up on the second monitor, but nothing else (background pic, windows).
Sometimes the screen flickers and I see hints of the desktop. I doubt there's something wrong with my cable because the monitor works in Windows.
Also this may be useful. My first monitor is connected to my motherboard (cpu graphics, 530) via DisplayPort and my second monitor is connected directly to my 1070 via DisplayPort.
I've tried Lilu and WG thinking maybe that'll fix something.