> There is a way to shut-down one cpu in OF: [...]
> Maybe OS notice that and start with one CPU.

As has been established some years back, this OF command just sets a flag that an OS may comply with (such as MacOSX) or may ignore (such as MorphOS).
As of MorphOS 3.10, the 2nd G5 CPU is disabled by default as opposed to pointlessly running all the time (and full-speed at that) as was happening with pre-3.10, but this has nothing to do with any OF setting.