IIRC you run a GTX 770 right? Nvidia will only run SLI with the same graphics chipset, so if you wanted a dual card solution you would have to get another of the same. So you will have to have 2 770's or 2 960's.
AMD will run crossfire with 2 different models, but the faster card will will slow down to the same performance as the slower card. In other words if you are running a r9 290 with an r9 280, the 290 will only run as fast as the 280.
How picky is the hackintosh?? will it only run with certain GPU brands? Or is it just the chipset that matters? I notice they dont have OSx drivers on nvidias website.