I say no.
I think the 260 would be slowing down the 680.
The 260 would be drawing the frames at a much slower rate than the 680 can, thus causing the 680 to slow itself down. resulting in lost FPS.
If you were to run the 680 alone I believe it would be able to render everything by itself at a faster FPS since the card is incredibly powerful.
However why not run a benchmark with the current config, and then again with just the 680. This way you will see for yourself and get a definitive answer.
I hope it helped.
*edit* this video will help explain what's going on here.http://www.youtube.com/watch?v=cbww3dhzK0M