did you manage to get both NVIDIA and AMD GPU working on Linux?
If so, could you provide details on how you did it?
I have three GPUs installed on my GNU/Debian machine: two AMDs (HD6970 and HD7970) and an Nvidia GTX-580. The HD6970 GPU is mainly used for display, while HD7970 and GTX-580 are exclusively used for GP-GPU computing through OpenCL.
I've installed the fglrx driver (12.3) and the nvidia driver (295-33) from Debian's repository. Then, I've configured Xorg to recognize the AMD GPUs by executing 'aticonfig --initial --adapter=all'.
In order to be able to start X I had to blacklist the nvidia driver (echo "blacklist nvidia" > /etc/modprobe.d/nvidia.conf), otherwise the fglrx driver hangs while trying to start X. Fortunately, this didn't stop the nvidia driver from being automatically loaded whenever some process (such as OpenCL programs) attempts to use the hardware.
I did a fresh install today with only Radeon GPU installed, then successfully loaded installed driver. After that I inserted GTX 470 back in the first slot (otherwise it does not fit in my case) -- the system continued to boot with nouveau driver and AMD-opencl applications remained operational. When I installed nvidia driver it stopped booting with the symptoms you'd described -- fgrlx hung while trying to start X. Blacklisting of nvidia driver did not help, so I ended up uninstalling nvidia drivers.
The only difference between our systems is that your first GPU is an AMD one, while mine is NVIDIA. I'm using openSUSE 11.4, but I don't think this matters.