[DEBIAN][DUAL-HEAD] Position des écrans

Bonjour,
je suis entrain de configurer mon bureau sur 2 écrans. Lorsqu'ils sont gérés indépendemment, je n'ai pas de soucis je sort de celui de gauche par la droite et inversement. Par contre, dans le cas du burea étendu, impossible d'arriver au même résultat!!!
Quelqu'un peut il m'aider SVP???

je vous mais mon XF86Config-4:

# XF86Config-4 (XFree86 X Window System server configuration file)
#
# This file was generated by dexconf, the Debian X Configuration tool, using
# values from the debconf database.
#
# Edit this file with caution, and see the XF86Config-4 manual page.
# (Type "man XF86Config-4" at the shell prompt.)
#
# This file is automatically updated on xserver-xfree86 package upgrades *only*
# if it has not been modified since the last upgrade of the xserver-xfree86
# package.
#
# If you have edited this file but would like it to be automatically updated
# again, run the following commands as root:
#
#   cp /etc/X11/XF86Config-4 /etc/X11/XF86Config-4.custom
#   md5sum /etc/X11/XF86Config-4 >/var/lib/xfree86/XF86Config-4.md5sum
#   dpkg-reconfigure xserver-xfree86

Section "ServerLayout"
	Identifier     "Default Layout"
	Screen      0  "aticonfig Screen 0" 0 0
	InputDevice    "Generic Keyboard"
	InputDevice    "Configured Mouse"
EndSection

Section "Files"
	# local font server
	# if the local font server has problems, we can fall back on these
	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/CID"
	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  "record"
	Load  "speedo"
	Load  "type1"
	Load  "vbe"
EndSection

Section "InputDevice"
	Identifier  "Generic Keyboard"
	Driver      "keyboard"
	Option	    "CoreKeyboard"
	Option	    "XkbRules" "xfree86"
	Option	    "XkbModel" "pc105"
	Option	    "XkbLayout" "fr"
EndSection

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

Section "Monitor"
	Identifier   "aticonfig Monitor 0"
EndSection

Section "Monitor"
	Identifier   "aticonfig Monitor 1"
EndSection

Section "Device"
	Identifier  "ATI Graphics Adapter 0"
	Driver      "fglrx"
	Option	    "(null)"
	Option      "DesktopSetup" "horizontal"
        Option      "OverlayOnCRTC2" "0"
	BusID       "PCI:1:0:0"
EndSection

Section "Device"
	Identifier  "ATI Graphics Adapter 1"
	Driver      "fglrx"
	BusID       "PCI:1:0:0"
	Screen      1
EndSection

Section "Screen"
	Identifier "aticonfig Screen 0"
	Device     "ATI Graphics Adapter 0"
	Monitor    "aticonfig Monitor 0"
	DefaultDepth     24
	SubSection "Display"
		Viewport   0 0
		Depth     24
	EndSubSection
EndSection

Section "Screen"
	Identifier "aticonfig Screen 1"
	Device     "ATI Graphics Adapter 1"
	Monitor    "aticonfig Monitor 1"
	DefaultDepth     24
	SubSection "Display"
		Viewport   0 0
		Depth     24
	EndSubSection
EndSection

Section "DRI"
	Mode         0666
EndSection

encore merci de votre aide!