Why not? Since you're only using half the power envelope, while most of your PC components are stressed, I don't see why your PSU couldn't support another gtx680 (TDP ~195W).
But I only tested for 20-30 minutes. I should do a bit more testing (more games, more situations, longer). But during that brief testing, I did see power usage spike over 300W during Crysis 3. Maybe up to 350W even. I tested only for 20-30 minutes, because I was already relieved that I wasn't getting close to 500W at all. Spikes with 2x gtx680x might get close to 500W. But it isn't important now.
I wasn't planning on doing SLI anyway. Cost would be too high, as I'd need to buy a 2nd waterblock. I rather wait till nVidia releases its 700 series of new cards. And buy a single top-end 700 card.
Edit: did some more testing.
Inspired by a review on SPCR, I ran Furmark in combination with Prime64. Just running Prime64 made my computer draw 150W from the wall. Starting Furmark on top of that would make the powerdraw go to 350-360W. I also ran IntelBurnTest on top of that, but that hardly made a difference. (I guess Prime64 is enough to max out the CPU).
So the videocard seemed to be able to use 200-210W. When I'm already at 360W, a 2nd gtx680 would make my maximum power increase to 570W. That's over the max. Maximum is 500W to components, at 95% efficiency (at 230V), which gives the number 526W. A second gtx680 could cause my power consumption 8.3% over budget.
Furmark is an extreme application. I guess under realworld scenarios, I would never use that much. And I would stay within 500W while playing games. But as I said, it would be pushing it. I rather overclock my gtx680, increase the powerbudget by 30%. And/or overclock my CPU a bit more (4.0Ghz->4.4GHz).
I also did the test to see how my cooling is doing.
I ran the test for 30 minutes. Room temp is 20C (estimate).
CPU cores all ran 65-70C. GPU ran at 43-47C. HDDs stayed at 26-28C.
Not bad, I think.