Borderlands 2 PhysX issues with dedicated card.

[Redheadsrule13]

I've been playing Borderlands 2 recently and I've discovered that PhysX chews up my framerate like crazy when I get into large fights.  I'm using a 760, which should be able to run PhysX just fine by itself, however I also added a gt640 I had for dedicated PhysX, and the problem still persists. I've looked around and seen some things like limiting the max number of PhysX particles, but it seems that my newer copy of Borderlands doesn't have those setting any more (probs got patched once people complained). I should be able to run the game butter smooth with my setup, but it lags all the way down to 15 fps with PhysX on high.
So, does anyone know a fix for this? I suspect for some reason neither GPU's are rendering the PhysX and it's actually my CPU, however the Nvidia overlay that tells you doesn't seem to want to work for me so I can't tell.

[Redheadsrule13]

So I did some testing, and using my 760 to render PhysX gave me the best FPS, and using the 640 or CPU ended up the same.
I also did a Fluidmark benchmark with each and got the exact same fps no matter what I used. Which makes me think that there's something weird going on.

[Drunken Savior]

I'm assuming you did, but I'll ask anyways just to cover bases:  You did make sure that your supplemental card was being used for PhysX in the NVIDIA Control Panel (not GeForce Experience), right?