BMT and (0i0i)5m4rt4l3X[-CH-] pointed out that the latest beta drivers, version 331.93, don't need the new fix anymore. The official profile is now using the correct SLI compatibility bits, so you'll only need my guide if you're using drivers older than 331.93.http://steamcommunity.com/sharedfiles/filedetails/?id=181710281
Updated the guide with a new fix that uses the official (but broken by default) profile.
*edit (21/10/2013 bis)*
It seems the official profile is broken and not working as expected. I've tried out the profile (as have "pnarciso" and "dynamite deluxe!", see their comments in this thread) and I get worse performance than the fix described in my guide.
With my fix, I get a silky smooth framerate that never drops below 60FPS (maxed out settings), while the official profile is barely able to keep the framerate above 40-45FPS.
YMMV though, and I still suggest you try out the official profile first before trying my fix.
nVidia finally released an official SLI profile in the latest Geforce drivers (version 331.58). This profile is your best bet to get the best results on an SLI-enabled computer.
You can find more info and download links here: http://www.geforce.com/whats-new/articles/nvidia-geforce-331-58-whql-drivers-released
If you can't (or don't want to) install the latest drivers and would still like to get SLI working on an older set of drivers, be sure to read the guide below.
Created a guide (link see above)
I did some experimenting and found a simple fix to enable SLI. You'll need nVidiaInspector to make some changes to the existing Hard Reset profile, the previous game from Wild Hog. Both games use the same engine, so I decided to try adding Shadow Warrior executable to the Hard Reset profile with nVidiaInspector.
And it worked! Before this tweak, I had to run the game on High with SSAO disabled to get a solid 60FPS. This little change enabled me to run the game on Ultra settings with a rock solid 60FPS on my GTX580 SLI setup.
1. Download nVidiaInspector here: http://www.guru3d.com/files_details/nvidia_inspector_download.html
2. After downloading, run nVidiaInspector, and click on the "Driver profile settings" button (little button with the wrench and screwdriver icon next to your current driver info)
3. Look up the Hard Reset profile
4. Click on the "Add application to current profile" button (6th button on the top with the little green plus sign on)
5. Locate the Shadow Warrior exe, and add it. You will need to choose "Application absolute path" option next to the filename box. This is needed because another profile already uses the "SW.exe" filename
6. Save by pressing "Apply changes"
7. Play the game with SLI enabled!
This should help out anyone that uses SLI until nVidia releases an official profile for the game.