Just plug them both in and when you download the newest drivers it will install both. Then under the Nvidia control panel just pick the 560 as the physx. It is quite simple. Now for the real answer ... You don't need the 560 in your system at all it will just heat up your room and not be doing anything 98% of the time. The 660 can do physx just fine by itself and you won't have to have a 700+ watt power supply to run all of that.