1) NO.
SLI requires the same GPU. You can go to NVidia's site for more information but basically you'd need 2xGTX760, or 2xGTX780 etc.
2) PHYSX:
While it works in theory, you can easily run SLOWER by using an addon card to process the physics. If it's not fast enough the main GTX760 would just end up waiting.
Linus from Tech Tips tested this and I think you'd actually SLOW DOWN your system based on his results. It also varies by the game and some games even have LOW, MEDIUM and HIGH levels of PHYSX (so LOW might benefit, but HIGH might be slower).