One card for physx the other for the graphics. Cpu is slower. plus it has to do lots more as well as the physx when playing a game. AI ect.
so are you saying i should use auto mode ?
yup. with SLI choosing auto will put the physx processing on second gpu.
Actually, putting PhysX to Auto will spread the PhysX processing out between both cards, allowing you to retain your full SLI performance. The Auto setting does not leave you with only one card doing graphics... that would just be silly. The important thing is that it does not revert to the CPU, which is the worst-case scenario.