they don't have to be exactly the same but they have to be the same model with the same memory amount ex. GTX768+ GTX768, GTS 640+ GTS 640, etc. They don't have to be from the same manufacture either and they don't even have to be running at the same speed as one can be an OC version while the other is not so long as they are the same model. Keep in mind that by default if one runs faster than the other, they will both link up at the slower of the two speeds. You can fix that by using Rivatuner or similar to manually OC both back up to a common higher speed. If you're brave you can also flash the bios of the slower one to that of the faster one and then they will both detect as the same speed so long as the slower card can handle the OC.