I'm stepping in here for a dual monitor problem I have, too. My G5 has got a Mac Radeon 9600 (RV360) with AGP and 1xADC and 1xDVI connector.. In case a Cinema Studio display is connected to the ADC connector, the display at the DVI port (BenQ FullHD) isn't recognised anymore. In the prefs I get only the screenmode for the ADC display, no ".1" screenmodes.

I guess the gfxcard isn't supported for dualhead support?