I know this is an old thread but I had to comment and correct the above.
Using a second card as a physx card would never create a bottleneck.
And also by unloading your primary gfx card from the heavy lifting that is physx(consumes masses of Vram) and outsourcing it to say a "gtx 645" you would allow your primary gfx to get on with everything else and no suffer big fps hits from using physx on a single card.
I'm using a gtx 970 G1 and with all physx options on in batman Arkham knight it shoots it straight over the max Vram usage so I have to back shadow detail down to keep the fps high; solution... a dedicated physx card