Originally Posted by Nightwolf
Based on your screenshot, you will want to configure Physx to run on the cpu. I'm thinking with Physx configured to run on the second GTX 460, this is why you can't enable SLI.
I have Physx running on my cpu in my own system.
Setting Physx to automatic will select the best option for your system. It doesn't use the card just for Physx unless you select the card in the drop down and check the "dedicate to Physx" box.