I got xinerama to work with my Radeon 8500, here's my config file. It works pretty well although I just changed some setting I can't remember and it locks up my computer. Oh well. I'll fix it eventually. This should work (just change the PCI Bus and monitor refresh rates.

Section "Files"
FontPath "unix/:7100"
FontPath "/usr/lib/X11/fonts/misc"
FontPath "/usr/lib/X11/fonts/cyrillic"
FontPath "/usr/lib/X11/fonts/100dpi/:unscaled"
FontPath "/usr/lib/X11/fonts/75dpi/:unscaled"
FontPath "/usr/lib/X11/fonts/Type1"
FontPath "/usr/lib/X11/fonts/Speedo"
FontPath "/usr/lib/X11/fonts/100dpi"
FontPath "/usr/lib/X11/fonts/75dpi"
EndSection

Section "Module"
# Load "GLcore"
Load "bitmap"
Load "dbe"
Load "ddc"
# Load "dri"
Load "extmod"
Load "freetype"
# Load "glx"
Load "int10"
# Load "pex5"
Load "record"
Load "speedo"
Load "type1"
# Load "vbe"
# Load "xie"
EndSection


Section "InputDevice"
Identifier "Generic Keyboard"
Driver "keyboard"
Option "CoreKeyboard"
Option "XkbRules" "xfree86"
Option "XkbModel" "pc104"
Option "XkbLayout" "us"
EndSection

Section "InputDevice"
Identifier "Configured Mouse"
Driver "mouse"
Option "CorePointer"
Option "Device" "/dev/psaux"
Option "Protocol" "ImPS/2"
Option "Emulate3Buttons" "true"
Option "ZAxisMapping" "4 5"
Option "Resolution" "500"
EndSection

Section "Device"
Identifier "Ati1"
Driver "radeon"
Option "crt_screen"
BusID "PCI:2:0:0"
Screen 1
EndSection

Section "Device"
Identifier "Ati2"
Driver "radeon"
Option "crt_screen"
BusID "PCI:2:0:0"
EndSection

Section "Monitor"
Identifier "A90f"
HorizSync 30-100
VertRefresh 50-160
Option "DPMS"
EndSection

Section "Monitor"
Identifier "G790"
HorizSync 30-86
# VertRefresh 50-150
VertRefresh 60
Option "DPMS"
EndSection

Section "Screen"
Identifier "Screen1"
Device "Ati1"
Monitor "A90f"
DefaultDepth 24
DefaultFbBPP 32
SubSection "Display"
Depth 24
Modes "1600x1200"
EndSubSection
EndSection

Section "Screen"
Identifier "Screen2"
Device "Ati2"
Monitor "G790"
DefaultDepth 24
DefaultFbBPP 32
SubSection "Display"
Depth 24
Modes "1600x1200"
EndSubSection
EndSection

Section "ServerLayout"
Identifier "Desktop"
# Screen "Screen2" RightOf "Screen1"
Screen "Screen1" 0 0
Screen "Screen2" LeftOf "Screen1"
InputDevice "Generic Keyboard"
InputDevice "Configured Mouse"
Option "Xinerama"
EndSection

Section "DRI"
Mode 0666
EndSection