Actually you CAN use both of those at the same time, however not like you are invisioning. You can use the Nvidia card to run Nvidia Physx while using the Radeon card to do the work, but you cannot get them to run in crossfire or sli together.
It has been a while since I have seen anything about people doing it, but if I remember properly there was quite a bit of tweaking to get it to work, much more of a pain than it is worth if you ask me.