With modded drivers it would be possible to have an Nvidia card be dedicated to Physx calculation while you use your two 6970's for graphics processing (Needs modded driver in that case because Physx automaticaly disables when an AMD/ATI card is detected in the system, even if theres an Nvidia card present meant for Physx processing, itl still be locked out, only way to use AMD/ATI card/s for primary graphics processing and have another Nvidia card for Physx is with the use of modded drivers.)
But since you do not have an Nvidia card im not sure if theres a way for you to use Physx, ive heard of people forcing it to be calculated by the processor instead of GPU, but the way Physx is programmed, it will not use all resources from the processor so the performance will be extremely bad and not worth it, not to mention it will not show the "full" physx effects that can be seen with it done by a CUDA capable Nvidia card.
Only way to get smooth and full Physx experience is with a capable Nvidia card im afraid.