Nvidia driver by default automatically determines when to use the second card and when not to, depending on the optimisation of whatever application you are running. You can work around this by going into nvidia driver and manually force alternate frame rendering (try either mode). This should ensure equal work load for each card and is a good way to test if your SLI setup is working correctly. From my experience however, you will get the best performance simply leaving it for the driver to decide.
1. Just go into Nvidia control panel.
2. On the left, go to 3D Settings --> Manager 3D Settings
On the right, click on the "programs settings" tab, or you can leave it on "Global Setting" if you want to use this setting for every game (highly not recommended).
3. Beneath that, under "Select a program to customise", find your game in the drop down.
4. Scroll down to the feature "SLI Rendering Mode", and change the setting by selecting either "force alternate frame rendering 1" or "force alternate frame rendering 2".
5. click ok, apply and exit, and run your game.
This usually gives you lower frames than leaving it on "nvidia recommended" in my experience, but you can try both mode 1, 2 and auto to see which one's the best.Edited by Sashimi - 12/5/12 at 6:24pm