G4 Powerbook mirroring

Using code and a setup that normally works correctly for the G3
Powerbook, I'm having trouble getting monitor "mirroring" to work on
the G4 Powerbook. All the Matlab and Psychtoolbox functions appear to
work normally (rebooted in OS 9.2), but when I try and mirror the
image to the projector, it complains about their being two screens {0
and -1}. What's the trick?