Caption says it all. I've had my GTX 590 for about a year now and decided to reinstall crysis - when I set the Multi-GPU mode from "Custom (Nvidia recommended)" to "Single-GPU" There was either no difference in frame rate, or the difference in frame rate was incredibly small.
You would think that Crysis, of all games, would have the best optimization with regards to SLI - Why is there so little change between multi-gpu and single-gpu in Crysis?
PS: I'm talking about Crysis 1 btw. Haven't played either Warhead or 2.
So basically, an underclocked 580 vs two underclocked 580s...
What about the rest of your specs? I know with a beast of a card like the 590 you should be blowing Crysis away.
And +1 to what the user above said. Make sure Crysis is patched and also make sure your drivers are up to date.