> can a Dual Processor PowerMac G5 startup with MorphOS
> with just one CPU fitted inside the tower, or does it have to
> have 2 working CPUs for MorphOS to switch one into some
> sort of "standby" mode, or some other option(s) since v3.10?

Going by koszer's comment #10 it seems to depend on the specific dual-G5 PowerMac model. If it's a dual-G5 model that requires 2 CPUs installed to work, the OS can't override this. It's enough if just one CPU works while the other is broken (yet installed). In this case, some OS (MorphOS?) may require the working one to be installed in the primary slot, though. If it's a dual-G5 model that can work with 1 CPU installed, any OS will work in this config. Also here, some OS (MorphOS?) may require the CPU to be installed in the primary slot, though. At least that's how I understand it anyway :-)
Regarding the MorphOS 3.10+ feature of disabling the 2nd CPU, I guess this feature simply does not become effective in case of non-existent or broken 2nd CPU.

> if one CPU/heatsink can be removed to save it from being run without any
> benefit to the OS (MorphOS) [...] then that might be a bonus, saving one
> of the working CPUs for the situation where the other one fails in the future

This sounds reasonable, assuming your dual-G5 model can work with 1 CPU installed. The MorphOS 3.10+ feature of disabling the 2nd CPU might not be too far behind physically removing the CPU with regards to preserving service life, though.