I have not seen anything that suggests the brand matters if the cards are the same. Make sure they are both running the same clock speeds. Keep a close eye on the temps too, as different manufacturers use varying heat shrouds. I would test them independently in your system to make sure the hottest one stays on top. You don't want the hotter of the two on the bottom cooking the top one.
And if you aren't already, make sure you are cooling your CPU properly. Multiple GPU's tend to bring case temps up considerably more than just a single GPU.