>>> you can deactived one processor too with some comand with morphos
>>> […]
>>> https://morph.zone/modules/newbb_plus/viewtopic.php?topic_id=13494&forum=11#155209

>> Do you know the command to deactivate the second processor?

> setenv boot-args cpus=1

No, that one just works for Mac OS, as is stated in comment #4 in the thread you linked to.
According to the release notes, MorphOS since 3.10 disables the 2nd G5 CPU on PowerMac7, as is stated in comment #6 in the thread you linked to. There have been some as yet unclarified uncertainties as to what "disabled" means in this context.