Using X :2 -configure, I found out Xorg was using Card0 as the device name to configure the graphics driver:
Section "Device"
Identifier "Card0"
Driver "radeon"
Option ....
EndSection
I also tried using Radeon as the Identifier and that worked too. Where does the value of Card0 come from? In other words, is that a default value for the first graphics card found? And why does Radeon also work in its place?