I don't think running them in SLI is possible, unfortunately. They're both running the same GPU, yes, but the rest of the cards are completely different, including memory speeds, etc, which typically need to be identical for SLI to work even with the same card. http://linustechtips.com/main/topic/24121-how-well-will...
I know but, not even if i flash the bios to a 770? The memory will work in the same speed
I could very well be wrong, but everything I've learned so far tells me no, it wouldn't work. And it's possible that trying it you could paper-weight your 680. Would be better to sell the 680 and get another 770, imo.
Edit: Also, it's not just about flashing the BIOS for the memory speed, either. The two cards are built differently. The GPU itself is the same (the GK104), but that's about it. There's no guarantee that the 680 would safely run the 1 GHz faster memory speed that the 770 is.