Not all G4s support idling, AFAIK everything QuickSilver or older completly lacks such features (and the fans aren't SW-controllable either). Not sure bout MDD/FW800s.
But don't dispair, that feature runs quite well on Minis,PowerBooks and iBooks where the only CPU is throttled down to keep the fans quieter when idling. So at worst we would have the 2nd G5 tucking along at 10% of it's max speed where it wouldn't do much damage....