With the drivers that are current as of this post date, there is no SLI profile for Second Life. That means even if you have an SLI system, Second Life will run in single CPU mode. That means one of your graphics cards sits unused while you play Second Life.
My gaming rig has dual 7900 GTX cards. I can't stand the thought of a $600 graphics card sitting unused, so I decided to test the various SLI rendering modes myself. Your mileage may vary, but I hope this gives you a jumping point in setting up a profile for Second Life on your SLI system.
First, it may be handy to read nVidia's guide on creating an SLI profile. You'll notice there are basically 4 SLI options: Single-GPU (SLI off, in essence), Split Frame, Alternate Frame 1 and Alternate Frame 2. Split Frame produced bizarre rendering problems for me and Alternate Frame 1 lowered the framerate and causes severe flicker of all 2D interface elements. That left me very concerned as to the possible success of Alternate Frame 2.
Luckily, AFR2 does the trick. There's no rendering artifacts or flicker. I've always believed Second Life was very CPU bound so I didn't expect great framerate boosts from SLI, but my max framerate has doubled, and my average and minimum framerates have nearly doubled. That means I can walk onto a busy sim, with all the eye-candy on at 1600x1200 and still stay at a steady 30 frames per second.
I love SLI.