Very strange I typed the "setenv boot-args CPUS=1" but without the
"reset-all", because I don't know exactly what will be reseted, and I'm a
bit of a "Schisser";)
Then I type "mac-boot" to continue booting, but the cd tray was pushed out,
I manually pushed it back, and no booting so I long pressed the reset
button until to shutdown the G5.
Then pushed it again to boot it, but this time I didn't hear the
click-clack of the cd drive and no booting.
So I switched it off again and on but pressed "c" to open the cd tray, but
this time pushed the hidden eject button of the cd drive to insert the tray
with no cd.

Then booted again with alt-key pressed to open the boot-menu and ctrl-y (on
a german keyboard) to go into the firmware.

There I checked the cpus, with "dev /cpus" and "ls", both cpus were shown!
Then I did "dev /cpus/PowerPC,G5@0" and ".properties".
State was "running".
Then I did "dev /cpus/PowerPC,G5@1" and ".properties".
State was here "stopped".

Note "boot-args" was still set to "CPUS=1".

Then I dared to do "mac-boot" and MorphOS booted again in full beautyness!

BTW: I have a PowerMac 7,3 with 2,3GHz.

Lets see how long it works....