Supporting Incompatible Graphics Cards

Only if there are drivers for those cards. That should be checked manually for every card…

For example, NVidia published web drivers for Pascal-based GPUs for High Sierra, but haven’t done that yet for Mojave (which delays further installation of Hackintosh Mojave for me since I have GTX 1060 in my laptop).

And I’m not entirely sure, does your card support Crossfire (assuming they have same chips) in Mac OS, that should be checked, too… ( I remember there was Mac Pro with Dual AMD GPU setup, but that’s the only one…)

As well as that the second GPU you’re about to install has the same chip as the GPU installed into your Mac tower right now. Because if they’re not, you’re probably out of luck.