Example: If I had an MSI GTX 970 and an EVGA GTX 970, would I be able to put them on the same motherboard and use SLI? I know you can't do it with different GPUs (e.g. R9 390X and GTX Titan Black) but what about different brands of the same ones?
You can do SLI/crossfire with different brands of the same GPU and if they have different clock rates ( 1.15GHz vs 1.05GHz for example), the higher clocked card will downclock to match the slower card to ensure that they are both performing the same
Short answer, yes you can. You want to keep the clock speeds close to the same though or be able to oc one to reach the other as they will run off the lowest cards clock speeds if you don't change it in afterburner to unlink them. I'm pretty sure you can unsynch them as far as clockspeeds but you have to do it with afterburner, don't the last part about unsynching them as absolute truth but I'm pretty sure you can.