After a lot of trial and error, the conclusion is that it is not possible to enable both devices in Windows at the same time (haven't tried Linux).
The hardware in question is Biostar A75MH motherboard with the A4-3300 APU and 8800 GTS nVidia GPU.
The only setting that allows for APU to even be detected is "Force Enable" APU (detection of nVidia disables onboard graphics by default). The option for "Surround View" in BIOS must also be enabled, or the machine will not even boot..
Once in Windows (Vista 64-bit),
- if nVidia is primary display, the APU driver fails and is disabled.
- if APU is primary display, nVidia driver fails and is disabled.
Basically one steps on toes of another. Quite disappointing.
PS. The error code is actually "Code -43".
vista doesn't support two video drivers at same time. use windows 7