First off, lets make sure you have SLI installed properly.
Things you need:
1 SLI link cable
2 Geforce Video Cards
1 SLI Capable Motherboard.
1 SLI Capable Power Supply. a 800 or maybe a 750 will handle what your gonna do, look at my set up.
Now:
Turn off computer. Unplug from wall outlet. Make sure there is not power, not even to the mothboard, make sure the green Mobo light is off. Now take the side panel off. move all cables away from the PCI express slots if necessary (Only if you have a cable clutter in your case) Install your dual fan card in the first PCI Express slot, because otherwise temps can tend to get a little hot. your lower card most of the time will be the coolest.
Now once you have plugged your first card in, find your SLI cable and have it ready. now plug in the 2nd identical ( Different brand same exact Chip and Same exact ram amount) I prefer to use the same brand cards but when its not an option.
Unbox your 2nd video card and put plug it into the next PCI Express slot (Try using a X16 or a X8, an X4 is not recommended)
Now, take your SLI link cable and plug it into the SLI Pins on the side of the video card facing you. Make sure the link is snapped in properly, if you have both video cards in and get into windows, later and you don't have a SLI option in the NVIDIA control panel, then the cable isnt plugged in properly.
Now your ready to plug these cards into power, Take the PCI Express 6 Pin power connectors and plug them into your cards. Again make sure your Power Supply has enough power and the needed Power Dongles to use those cards.
Now that everything is plugged up and plugged in. return to steps 1 and 2. Plugg the computer back into your wall socket, and make sure your power supply is flipped off when you do step 3. Even tho your motherboard is powerered off. Capacitors in the Mobo and Power Supply still hold energy, tho it may not be enough to kill you. well you just don't wanna find out.
Flip the power switch to the power supply to make sure everything is recieving power again. wait a second or to for power to surge through. now your ready to Switch the case on. Now make sure your video card is plugged into your monitor.
Once in windows. Go to
http://www.nvidia.com/Download/index.aspx?lang=en-us
To get the latest drivers for SLI and everything else.
Now once the driver has been installed. you may need to restart your computer, if not go to NVIDIA control panel. go to set SLI and Physx configuration. Then click Maximize 3d performance. and you are now SLI'd.
I hope this guide has helped, if you need any more assistance or information. Just send me a Private message or just ask on here