And make sure they have the same VRAM. If they both don't have the same ammount, you will have issues. If you've got a 2gb card, buy another 2gb card. Same rule applies with the 4Gb option.
And they can be TOTALLY different vendors, and you can still SLI them... You could even buy a EVGA or ASUS 760 and SLI them. It simply has to do with the GPU model, as long as they're the same.