This was an interesting question, though they are similar they have been modified in ways that they are unable to SLI together.
Reason being the GTX 770 is an updateded version of GK 104, whilst the 680 is the standard released GK 104, it won't work, and if you somehow did, it would be chaotic.
If you are brave enough to flash the bios of your 680, to make it think its another 770, I wouldnt suggest it though as you might end up with a bricked 680.
This was an interesting question, though they are similar they have been modified in ways that they are unable to SLI together.
Reason being the GTX 770 is an updateded version of GK 104, whilst the 680 is the standard released GK 104, it won't work, and if you somehow did, it would be chaotic.
NO ! you need to have the same cards if you want to sli GTX 680 goes with the GTX 680 and you can do a 4 way sli but when we start talking about the GTX 690 you can put only 2 of them because its a dual-gpu meaning quad sli and yes