ATI cards do not support physX of any kind, in any way shape or form at all. No go.
The hacks allow you to use a nvidia card as a physx card behind an ATI gpu. But it would be a really stupid thing to do in your case, use a 3850 as your main card and a 460 as a secondary physx card. The 460 by itself is twice the card as the 3850.
Agree'd, something along those lines, you would only get true performance if you had the 3850 running as the primary gpu, with something like an 8600gt running as the secondary for physx.
The 460 is a powerful card, so if you used it as a physx only card, then you wouldn't be doing much, and probably get worse framerate then the 3850 with the same game on medium settings.