hélas je n’ai pas de liens, et j’ai “galéré” à faire le mien (pas toujours simple de trouver les infos utiles au milieu de tout)
voila mon xorg.conf (peut être que ça pourra t’aider)
Section "ServerLayout"
Identifier "X.org Configured"
# Screen 0 "Screen0" 0 0
# Screen 1 "Screen1" RightOf "Screen0"
Screen 0 "Screen0" 0 0
Screen 1 "Screen1" 0 1
# InputDevice "Mouse0" #"CorePointer"
# InputDevice "Keyboard0" #"CoreKeyboard"
EndSection
Section "Files"
ModulePath "/usr/lib/xorg/modules"
FontPath "/usr/share/fonts/X11/misc"
FontPath "/usr/share/fonts/X11/100dpi/:unscaled"
FontPath "/usr/share/fonts/X11/75dpi/:unscaled"
FontPath "/usr/share/fonts/X11/Type1"
FontPath "/usr/share/fonts/X11/100dpi"
FontPath "/usr/share/fonts/X11/75dpi"
FontPath "built-ins"
EndSection
#Section "Module"
# Load "glx"
#EndSection
#Section "InputDevice"
# Identifier "Keyboard0"
# # Option "Device" "/dev/input/event0"
# Driver "evdev"
#EndSection
#Section "InputDevice"
# Identifier "Mouse0"
# Driver "evdev"
# Option "Protocol" "auto"
## Option "Device" "/dev/input/mouse0"
# Option "Device" "/dev/input/mice"
# Option "ZAxisMapping" "4 5 6 7"
#EndSection
Section "Device"
Identifier "Card0"
Driver "radeon"
BusID "PCI:2:0:0"
Option "Monitor-HDMI-0" "HDMI0"
Option "Monitor-VGA-0" "VGA0"
EndSection
Section "Monitor"
Identifier "HDMI0"
VendorName "Monitor Vendor"
ModelName "Monitor Model"
Option "dpms" "true"
# Option "Primary" "true"
EndSection
Section "Monitor"
Identifier "VGA0"
VendorName "Monitor Vendor"
ModelName "Monitor Model"
Option "dpms" "true"
Option "LeftOf" "HDMI0"
EndSection
Section "Screen"
Identifier "Screen0"
Device "Card0"
Monitor "HDMI0"
DefaultDepth 24
SubSection "Display"
Depth 24
Modes "1920x1080"
EndSubSection
EndSection
Section "Screen"
Identifier "Screen1"
Device "Card0"
Monitor "VGA0"
DefaultDepth 24
SubSection "Display"
Depth 24
Modes "1024x768"
EndSubSection
EndSection
#Section "ServerLayout"
# Identifier "Layout0"
# Screen 0 "Screen0"
# Screen 1 "Screen1" LeftOf "Screen0"
#EndSection
il est loin d’être parfait, mais j’ai été au plus simple
par contre, quand je fais xrandr mes deux écrans sont reconnu, dans ton premier post il n’y en a qu’un 
debian@debian:~$ xrandr
Screen 0: minimum 320 x 200, current 2944 x 1080, maximum 8192 x 8192
HDMI-0 connected 1920x1080+1024+0 (normal left inverted right x axis y axis) 480mm x 270mm
1920x1080 60.00*+ 50.00 59.94
1920x1080i 60.00 50.00 59.94
1680x1050 59.88
1400x1050 59.95
1600x900 60.00
1280x1024 75.02 60.02
1440x900 59.90
1280x800 59.91
1152x864 75.00
1280x720 60.00 50.00 59.94
1024x768 75.08 60.00
800x600 75.00 60.32
720x576 50.00
720x480 60.00 59.94
640x480 75.00 60.00 59.94
720x400 70.08
DVI-0 disconnected (normal left inverted right x axis y axis)
VGA-0 connected 1024x768+0+0 (normal left inverted right x axis y axis) 0mm x 0mm
1024x768 60.00*
800x600 60.32 56.25
848x480 60.00
640x480 59.94