You seem to have SLi confused with using a second card as a PhysX processor.
As I said however, there's no guarantees this will improve your performance by a noticeable amount.
I think he may have been referring to ATI's method for Xfiring different GPU's.
EDIT: @ LJW10000: You cannot dedicate a CPU to PhysX, PhysX is a nVidia only GPU feature. and as for improved performance, Read above.