I just got 2 670s (one is the Gigabyte Windforce OC) the other is the Stock EVGA 670. Now I'm using MSI Afterburner to Overclock and it works Well, I tried OCing the EVGA to match the Gigabyte but then it also overclocks the Gigabyte by 100 mhz.
I tried switching don't synchronize in the settings tab, but it doesn't work:S
Also I noticed that with the Evga I can increase the voltage and the Power limit much more when it is in the main slot. Is it restricted by Gigabyte? If so...wow! Why sell a card that can clearly take an overclock and limit it .
For SLI to work both cards have to be operating at the same GPU clock and memory clock frequencies. You'll may have to over/under clock the cards so that they are both using a GPU clock and memory clock that are stable for both cards.