I finally managed to get my dual monitor working, even though it was a bit tricky for a newbie like me, as I had to resort to the terminal :)

I installed the AMD Catalyst Control Center (via Synaptic package manager or Software Center)

This resulted in adding 2 commands in the LXDE menu, under "Preferences"

1. "AMD Catalyst Control Center", i.e., amdcccle

2. "AMD Catalyst Control Center (Administrative)", i.e., "amdxdg-su -c amdcccle"

Neither 1. nor 2. worked, so I tried "sudo amdcccle" in a terminal (no quotes), which displayed amdcccle graphically.....

And after a few fiddling with the GUI of Catalyst (Multi-display desktop with displays (1)), voila my wonderful 2 headed display!

Technically, this whole process modified the contents of /etc/X11/xorg.conf

I would recommend you backup this file manually in case something got screwed, which did happen to me several times :) while tampering with Xinerama (even though it looks like AMD Catalyst Control Center creates a backup)

So, to resume my case, originally xorg.conf merely contained:

[code start]

Section "Module"
    Load    "glx"
EndSection

[code end]

And after  the change by Catalyst:

[code start]

Section "ServerLayout"
    Identifier     "amdcccle Layout"
    Screen      0  "amdcccle-Screen[1]-0" 0 0
EndSection

Section "Module"
    Load  "glx"
EndSection

Section "Monitor"
    Identifier   "0-DFP1"
    Option        "VendorName" "ATI Proprietary Driver"
    Option        "ModelName" "Generic Autodetecting Monitor"
    Option        "DPMS" "true"
    Option        "PreferredMode" "1920x1080"
    Option        "TargetRefresh" "30"
    Option        "Position" "1920 0"
    Option        "Rotate" "normal"
    Option        "Disable" "false"
EndSection

Section "Monitor"
    Identifier   "0-DFP2"
    Option        "VendorName" "ATI Proprietary Driver"
    Option        "ModelName" "Generic Autodetecting Monitor"
    Option        "DPMS" "true"
    Option        "PreferredMode" "1920x1080"
    Option        "TargetRefresh" "60"
    Option        "Position" "0 0"
    Option        "Rotate" "normal"
    Option        "Disable" "false"
EndSection

Section "Device"
    Identifier  "amdcccle-Device[1]-0"
    Driver      "fglrx"
    Option        "Monitor-DFP1" "0-DFP1"
    Option        "Monitor-DFP2" "0-DFP2"
    BusID       "PCI:1:0:0"
EndSection

Section "Screen"
    Identifier "amdcccle-Screen[1]-0"
    Device     "amdcccle-Device[1]-0"
    DefaultDepth     24
    SubSection "Display"
        Viewport   0 0
        Virtual   3840 1920
        Depth     24
    EndSubSection
EndSection

[code end]

Hope this helps U 2