Install two incomptaible nvidia drivers at the same time in windows

Unfortunately, what you wish seems impossible unless you have some guru-level programming and OS knowledge. The GTX 970 driver only supports back to the 400 series. Basically nVidia would have to make a conscious effort to merge support of the 200 series into the current driver. I would imagine it would also increase the driver size to over 600 MB. Good luck in your quest but overall I too think that the PhysX delivered by the 280 will actually hold back your 970

There was a video I saw of a GTX 580 using a 8600 GTS for PhysX and the FPS drop was substantial vs. just running everything on the 580. I cannot find it though :(

^ I found the video! https://www.youtube.com/watch?v=cbww3dhzK0M the details

Here is a solid run-down: http://www.volnapc.com/all-posts/how-much-difference-does-a-dedicated-physx-card-make and this picture specifically http://www.volnapc.com/uploads/3/0/9/1/30918989/7654086_orig.jpg. Unfortunately the blog does mention using a 200 series with 900 series so I question the credibility.