Multiple GPUs in Mojave

I used to have a dual GPU setup in my computer and used that from Mavericks to Sierra. I started out with 2 GTX 660s but found it to be unstable, so I removed one and used an AMD Radeon HD 6450. Neither of those cards worked right since high Sierra, so I switched to an RX 560. I have 3 1440p DVI monitors and can only use one with the RX 560, which was alright for a while but I’d like to use all 3 again. The active DisplayPort to dual link DVI adapters are $70-100 each, but it looks like I can pick up a used AMD GPU or two for less than I would spend on the adapters that sound a bit questionable based on the reviews. And I’d need to buy a second GPU anyway.

Just for fun, I tried putting the GTX 660 back in there along with the RX 560 but when I boot with one monitor connected to each card, both are black. Does mixing GPU brands no longer work in Mojave? I saw a few threads where two matching cards supposedly worked. If I were to buy an R9 290x, could I expect this same behavior?

MSI Z87-GD65 gaming, i5 4690, Sapphire RX 560, iMac 14,2 SMBIOS