CE L'HO FATTA!!
Posto qui il mio xorg.conf nel caso potesse servire a qualcuno

cat /etc/X11/xorg.conf
############################
Section "Files"

RgbPath "/usr/X11R6/lib/X11/rgb"
FontPath "/usr/X11R6/lib/X11/fonts/misc/"
FontPath "/usr/X11R6/lib/X11/fonts/Type1/"
FontPath "/usr/X11R6/lib/X11/fonts/CID/"
FontPath "/usr/X11R6/lib/X11/fonts/75dpi/"
FontPath "/usr/X11R6/lib/X11/fonts/local/"
FontPath "/usr/X11R6/lib/X11/fonts/Speedo/"
EndSection
######################################
Section "Module"

Load "dbe" # Double buffer extension
Load "type1"
Load "freetype"
Load "GLcore"
Load "bitmap"
Load "ddc"
Load "extmod"
Load "int10"
Load "record"
Load "speedo"
Load "vbe"
SubSection "extmod"
Option "omit xfree86-dga" # don't initialise the DGA extension
EndSubSection

Load "glx"
Load "dri"

EndSection


# DRI Section
# ************************************************** ********************
Section "dri"
# Access to OpenGL ICD is allowed for all users:
Mode 0666
# Access to OpenGL ICD is restricted to a specific user group:
# Group 100 # users
# Mode 0660
EndSection

#########################################

Section "InputDevice"

Identifier "Keyboard1"
Driver "kbd"
Option "AutoRepeat" "500 30"
# Specify which keyboard LEDs can be user-controlled (eg, with xset(1))
Option "XkbRules" "xorg"
Option "XkbModel" "pc105"
Option "XkbLayout" "it"

EndSection
#############################################
Section "InputDevice"

Identifier "Mouse1"
Driver "mouse"
Option "CorePointer"
Option "Protocol" "ExplorerPS/2"
Option "Device" "/dev/psaux"
#Option "Buttons" "7"
#Option "Vendor" "Sysp"
Option "Emulate3Buttons""true"
Option "ZAxisMapping" "4 5"

EndSection

Section "InputDevice"

Identifier "Mouse2"
Driver "mouse"
Option "SendCoreEvents""true"
# Option "CorePointer"
Option "Protocol" "ExplorerPS/2"
Option "Device" "/dev/input/mice"
#Option "Buttons" "7"
#Option "Vendor" "Sysp"
Option "Emulate3Buttons""true"
Option "ZAxisMapping" "4 5"

EndSection


################################################
Section "Monitor"

Identifier "sam"
HorizSync 30.0 - 70.0
VertRefresh 50.0 - 160.0
Option "DPMS"

EndSection

Section "Monitor"

Identifier "sam2"
HorizSync 28.0 - 96.0
VertRefresh 50.0 - 76.0
Option "DPMS"
EndSection
##################################################



Section "Device"

Identifier "rad"
Driver "r128"
BusID "1:00:0"

EndSection

Section "Device"

Identifier "rad2"
Driver "ati"
BusID "0:09:0"

EndSection
################################################## ###
Section "Screen"
Identifier "Screen 1"
Device "rad"
Monitor "sam"
DefaultDepth 24
#SubSection "Display"
# Viewport 0 0
# Depth 8
# Modes "1280x1024" "1024x768" "800x600" "640x480"
# EndSubSection
# SubSection "Display"
# Viewport 0 0
# Depth 16
# Modes "1024x768" "800x600" "640x480"
# EndSubSection
SubSection "Display"
Viewport 0 0
Depth 24
Modes "1024x768" "800x600" "640x480"
EndSubSection
EndSection

Section "Screen"
Identifier "Screen 2"
Device "rad2"
Monitor "sam2"
DefaultDepth 24
SubSection "Display"
Viewport 0 0
Depth 8
Modes "1280x1024" "1024x768" "800x600" "640x480"
EndSubSection
SubSection "Display"
Viewport 0 0
Depth 16
Modes "1024x768" "800x600" "640x480"
EndSubSection
SubSection "Display"
Viewport 0 0
Depth 24
Modes "1024x768" "800x600" "640x480"
EndSubSection
EndSection

################################################## #################################

Section "ServerLayout"

Identifier "Dual Layout"

Screen "screen1" 0 0
Screen "screen2" 1 1
InputDevice "Mouse1" "CorePointer"
InputDevice "Keyboard1" "CoreKeyboard"
# Option "xinerama"

EndSection

################################################## ###################################

Section "ServerFlags"

#Option "Xinerama" "false"
#Option "clone" "false"
EndSection