Ok, to reply to myself:
I solved the problem by using the proprietary nvidia driver for the 8500GT instead of the open source nv driver. Now my graphical desktop on the 8500GT runs without being disturbed by heavy OpenCL load on the ATI card.
Getting both the propr. ATI and NVIDIA driver to work in parallel on different X servers in a multi-seat configuration is possible, you only have to take care of the glx module for the X server by hand (-modulepath is your friend when starting the X server) and of the GL library.
OpenCL works on both graphics adapters, and in addition CUDA works on the nvidia card. I will try to find out if both OpenCL platforms can be used in parallel from one application...