either change the BusID to the one you need or use both at the same time. Example config:
Identifier "Default Device"
(you have to change BusID and ZaphodHead options according to your cards)
In order to get 3D acceleration you have to turn off Xinerama. With Xinerama enabled you can move windows from a screen on the r500 to a screen of the r800 and vice-versa (but it's not accelerated anymore).