it would be better that they are identical companies, since some factories clock them differently and they run at different speeds. one could be overclocked by the factory and the other could be normal. the different speeds would screw around with SLi
At first when SLI came out, you had to have both cards of identical makes and brand, which made it an obvious drawback. When the ForceWare 80.xx drivers came out, nVIDIA fixed the problem by allowing SLI to run with both cards of different brands. So they still have to be the same model (i.e. 2x 7900 GTX's), but the brands can be different (i.e. Card 1 is eVGA and Card 2 is BFG Tech).