Upgraded from a 1060 to a Vega 56, dual-monitor no longer working

Really weird, annoying issue. Maybe somebody encountered this before.

About two years ago, I upgraded from my single 1080p to two Dell U2515H, 1440p displays (both connected via DP). That was back when I had the 1060. IIRC, running El Cap, everything just worked. Then, I tried upgrading to sierra and my system would flat-out refuse to show anything if I had the second display connected. Black screen. So I went back to windows for a while. Came back few months later, to see if high sierra would work. After installing the nvidia web drivers, it actually did. So I forgot about the issue. And just enjoyed my hackintosh.

Fast forward to yesterday. I got a new Vega 56, both as an upgrade to the 1060 and because of the compatibility. Installed it and woosh. Same issue. When I connect the second display, my primary one switches to a solid color - which, somewhat, matches my background. The second screen goes into power-saving mode. Booting with both displays connected results in a black screen, although the display remains active. Reconnecting the displays does not help.

So far I've tried upgrading clover, as well as all kexts/drivers. I also tried whatevergreen and various 'fixes' I read about. The issue persists.

If you guys know how to fix this - or at least why it's happening, let me know - it's much appreciated.