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This HOWTO assumes that you are using the opensource drivers for your video card (ATI or Intel). Nvidia and AMD proprietary drivers come with their dual screen components.

1. Setup your xorg.conf configuration file:

Edit it as root:
gksudo gedit /etc/X11/xorg.conf
(in GNOME) or
kdesudo kate /etc/X11/xorg.conf
(in KDE)

2. Create a subsection under the “Screen” section with the following.

SubSection "Display"
Depth 24
Modes "1440x900" "1440x900" #the resolutions of your monitors
Virtual 2880 800

Xrandr will function without these xorg.conf lines, but it will limit the size of your desktop.

That's for a 1440×900 resolution. If you have a different resolution, put those resolutions under “modes” and then combine the width of both for “virtual”. In other words, if you have a 1280×800 monitor, it would be “Virtual 2560 800″.

3. Save your changes, exit and restart X.

4. Now, you can setup your dual screens however you like. First find out some information about the screens. Open a terminal emulator and type:
xrandr -q
It should tell you the name of each screen and its current resolution. On my computer, my screens are “DVI-0″ and “DVI-1″.

5. To make one large desktop including both screens, run this command:
xrandr --auto --output DVI-0 --mode 1440x900 --right-of DVI-1
DVI-0 is on the right, and DVI-1 is on the left.

6. Create a script called “startxrandr” to run this command whenever you want.
xrandr --auto --output DVI-0 --mode 1440x900 --right-of DVI-1

7. Make sure to make the script executable
chmod a+x startxrandr

There are also a few GUI applications that allow you to control xrandr. Check with your Linux distribution.

You can set xrandr to run when your display manager or desktop environment starts. See the its documentation for startup applications.

KDE and GNOME will automatically configure two screens correctly so that maximizing a window will only fill one of the screens. Some desktop environments do not yet have support for this. You will have to test the one you use. You should have no problems with Compiz.

[via maketecheasier.com]