To get back to this; I tried dual head with xf86-video-ati-6.8.0 today, which resulted in "Server caught signal 11, aborting."
But not before printing
to the log.
RADEON(0): Direct Rendering Disabled -- Dual-head configuration is not working with DRI at present.
So, again, is this a hardware (R420) limitation or a software one? If the latter, where is the limitation (DDX, DIX, ...) and what constitutes the limitation (i.e. what is needed to get it working)?
I'm trying to read up on the X server but it's really difficult to know where to start looking without some help. I hope the R200 docs are released soon, with the "beginner intro" I hear they contain!