Thing's simple, I'm running 680's in SLI and that's working great for me with everything that has multi-gpu support. Crysis 3 has that, however when I run Crysis 3 the second gpu isn't active at all. I'ts on 0% load whilst the first card runs at 95-99% load constantly. I've looked at several solutions for this but none of them have worked so far.

What I've done:
Re-installed latest drivers
Chosen the Crysis 3 profile in Nvidia control panel
Chosen the Crysis 3 preset in Nvidia Inspector
Created a config file with the multi-gpi setting, tried settings 1 and 2 (2 gpu and auto)
Applied eVGA's "SLI Fix"

I've read that one needs to remove the preset from nvidia control panel before the nvidia inspector one can work, however as that specific profile in Nvidia control panel is recommended by Nvidia, it cannot be removed. It says so in the tooltip, quite clear actually. Thus I've not been successful in removing it. May this be a hint towards what the issue is?

I'm using the Nvidia inspector and similar programs to see whether the second card loads or not.

Does anyone have any ideas?

What driver version are you using? Latest doesn't mean much since they update frequently.

I have GTX 680 sli as well and with driver version 314.07 WHQL both of my gpu's are working at 99-97% except sometimes (pretty frequently) when I look around in certain areas and my gpu usage on both gpu's goes down to 40% and my frame rate plummets.