No, just leave both cards plugged in. Nvidia control panel has an option for PhysX stuff. It'll let you either choose which card will do the PhysX, or else you can leave it on Automatic. For me, Automatic selects the 750 ti for all PhysX processing, and the normal 3D rendering goes to my main card. It really helps in PhysX games for preventing frame drops during large particle explosions and things like that.
I ran Nvidia's PhysX bench mark when I first put mine in, and the results were really good. Here is what I got:
https://drive.google.com/folderview?id=0B0DnoLWfOUZxbHVBQnh5N3ZRS2c&usp=sharing
780 ti alone was 79.3 average fps and 104.6 max fps.
Dedicating the 750 ti to PhysX gave 122.1 average fps and 218.5 max fps.
When I ran it, I noticed that the scenes without PhysX explosions ran the same. It was just that the explosions and stuff didn't slow anything down at all. The PhysX card (750 ti) just went from like 2% to 50% during those times, but the main card didn't take on any extra work.
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