I have a Dell OptiPlex 360 workstation at work, with 2 x ATI RV280 [Radeon 9200 PRO] graphics cards installed, which are attached to two identical 19" HII flat panel monitors. I'm using the open source Radeon driver with Ubuntu, and the proprietary drivers with Windows. The good news is that dual head configuration works for both OSes. The bad news is, I have to use a different hardware configuration for each OS to achieve this.

Hardware config #1: Dual monitors work for Windows XP Pro like this:

First display -> external VGA port
Second display -> DVI input on gfx card

Hardware config #2: Dual monitors work for Ubuntu 10.04.1 like this:

First display -> VGA port on gfx card
Second display -> DVI input on gfx card

I connected up the displays according to Config #2 and booted up Windows, which resulted in a mirror image on both screens. I was unable to login, as the login box was not visible. I unplugged the VGA lead from gfx card and plugged it into the external VGA port (Config #1) - Windows dual head works again, but the VGA-connected screen is not recognised by Ubuntu and remains in standby mode. Is it possible to configure a dual head setup for Ubuntu using Config #1, or am I missing something?

I tried setting up dual monitors using Config #1, this morning which didn't work. By default, there is no xorg.conf file in Ubuntu 10.04.1, so I generated one using:

$ sudo X :2 -configure

X.Org X Server 1.7.6
Release Date: 2010-03-17
X Protocol Version 11, Revision 0
Build Operating System: Linux 2.6.24-27-server i686 Ubuntu
Current Operating System: Linux harrier 2.6.32-24-generic #42-Ubuntu SMP Fri Aug 20 14:24:04 UTC 2010 i686
Kernel command line: BOOT_IMAGE=/boot/vmlinuz-2.6.32-24-generic root=UUID=a34c1931-98d4-4a34-880c-c227a2936c4a ro quiet splash
Build Date: 21 July 2010  12:47:34PM
xorg-server 2:1.7.6-2ubuntu7.3 (For technical support please see http://www.ubuntu.com/support)
Current version of pixman: 0.16.4
    Before reporting problems, check http://wiki.x.org
    to make sure that you have the latest version.
Markers: (--) probed, (**) from config file, (==) default setting,
    (++) from command line, (!!) notice, (II) informational,
    (WW) warning, (EE) error, (NI) not implemented, (??) unknown.
(==) Log file: "/var/log/Xorg.2.log", Time: Mon Sep 13 10:02:02 2010
List of video drivers:
    apm
    ark
    intel
    mach64
    s3virge
    trident
    mga
    tseng
    ati
    nouveau
    neomagic
    i740
    openchrome
    voodoo
    s3
    i128
    radeon
    siliconmotion
    nv
    ztv
    vmware
    v4l
    chips
    rendition
    savage
    sisusb
    tdfx
    geode
    sis
    r128
    cirrus
    fbdev
    vesa
(++) Using config file: "/home/michael/xorg.conf.new"
(==) Using config directory: "/usr/lib/X11/xorg.conf.d"
(II) [KMS] No DRICreatePCIBusID symbol, no kernel modesetting.

Xorg detected your mouse at device /dev/input/mice.
Please check your config if the mouse is still not
operational, as by default Xorg tries to autodetect
the protocol.

Xorg has configured a multihead system, please check your config.

Your xorg.conf file is /home/michael/xorg.conf.new

To test the server, run 'X -config /home/michael/xorg.conf.new'

ddxSigGiveUp: Closing log

$ sudo X -config /home/michael/xorg.conf.new

Fatal server error:

Server is already active for display 0

    If this server is no longer running, remove /tmp/.X0-lock

    and start again.

Please consult the The X.Org Foundation support

     at http://wiki.x.org

for help.

ddxSigGiveUp: Closing log

I then booted Ubuntu in failsafe mode, dropped into root shell, and executed $ X -config /home/michael/xorg.conf.new again. The screen went blank and turned off, so