In OpenFirmware you may see some information about your CPUS:

dev /cpus
dev PowerPC.G5@0
.properties

gives you information about CPU_0. The CPU should have status "running"

dev /
dev /cpus
dev PowerPC.G5@1
.properties

gives you information about CPU_1. The CPU should have status "standby" (or anything like this)

If CPU_1 is running, while CPU_0 sleeps, OF works, but OS may have problems. Then you have to change CPUs.
Notice: only MOS works with only one CPU. I'm not sure if other OS can switch down to 32bit.