Whether you're in trouble to setting up dual/multiple/parallel monitors on ubuntu or something like use ubuntu for giant screen, I hope this guide will make the process relatively quick and painless. Tto perform dual/multiple monitors on ubuntu go to System -> Preferences -> Display, the action above will display the following window.
 
Ubuntu Display Preference
Note carefully the window that appears :
  • If you see only one monitor, but you have more than one monitor installed 'click on Detect Monitor', it's should detect all the monitors connected to your Ubuntu.
  • When you enabled 'mirror screens', both the monitors will show the same display, you can disable it if you want.
  • Highlight the first monitor and click the ‘On’ radio button. if you want activated the first monitor.
  • Select the Show displays in panel check-box, which will display the icon to configure the display on the top right hand panel for easy access.
  • To change the display order of the monitors, simply drag the monitor to re-arrange them accordingly
  • You can also change your monitor rotation, Click 'Rotation' to rotate your monitor 180° or 90°.
You can (also) perform ubuntu dual/multiple/parallel monitors using third party driver/application, such as:
  •     Xinerama (open source and binary driver)
  •     nVidia-only TwinView (binary driver)
  •     ATI, Matrox MergedFB (open source driver)
  •     ATI-only BigDesktop (binary driver)
All options above perform the same function. However, because of the variations of graphics cards in the market,sometimes give various result.
Feature Differences:

Xinerama is an extension to the X Window System which allows applications and window managers to use the two, or more, physical displays as one large virtual display.” --Wikipedia. In other words, it allows X, as we'll call the X Window System, to use more than one monitor for display.       
  • Pros: Since Xinerama is an extension of X, most graphics cards, including Intel Integrated Cards and Matrox Cards, work well with it.
  • Cons: There are several known problems with Xinerama. It requires that the physical screens to have the same bit depth. Also in most implementations, OpenGL (3D) direct-rendering only works on one of the screns. Windows that should show 3D graphics on other screens tend to just appear black. However, the Solaris SPARC OpenGL implementation allows direct rendering to all screens in Xinerama mode.
TwinView is a mode of operation where two display devices (digital flat panels, CRTs, and TVs) can display the contents of a single X screen in any arbitrary configuration.” -TwinView Appendix G.
  • Pros: Twinview merges two screens into one, “tricking” X to think that there is only one screen. This always provide better OpenGL direct-rendering.
  • Cons: TwinView requires the binary nVidia Driver, which means only nVidia cards are supported.
Merged Framebuffer mode allows you to use 3D acceleration on both heads of a dualheaded Radeon card.” --MergedFB Wiki
  • Pros: Similar to TwinView, MergedFB merges two screens into one, therefore supporting 3D direct-rendering on both screens.
  • Cons: MergedFB only works with the open sources drivers (mga, ati, , which lack direct-rendering support for many Radeon models. MergedFB also limits 3D contexts to 2048x2048.
BigDesktop allows for dual screen control by merging the 2 screens into one single screen, allowing direct rendering on both monitors.

Common Step for Each HowTo:
These steps are essential for the HowTos to work, please follow them precisely.
All commands give should be executed within terminal.

BackUp File:
Perhaps the most important step of them all. Since we are going to be editing a system file that have the potential, in the instance of an accident, to disable your Xserver (aka No GUI/Mouse Function), it is EXTREMELY important to execute this command:
Code:
sudo cp /etc/X11/xorg.conf /etc/X11/xorg.conf_backup
The function of this command is to make a backup copy of your Xorg.conf file (which we'll be editing soon).

If for any reason, after the guide, you are unable to activate Xserver, execute the following command:
Code:
sudo cp /etc/X11/xorg.conf_backup /etc/X11/xorg.conf
Now, we always want to have a clean slate, because it makes the work much easier, and there is less of a chance that some other arbitrary item will mess this whole thing up.
So, unplug all your monitors except for one, then execute the following command:

Code:
sudo dpkg-reconfigure xserver-xorg
Just hit enter for the most options, though if you have the binary drivers installed, you might want to select that. This code will generate a new, clean, standard xorg.conf file for us to work on.
Now, choose the method with which you wish to achieve Dual-Monitor capability:

    Xinerama: http://www.ubuntuforums.org/showthread.php?p=1773624
    TwinView: http://www.ubuntuforums.org/showthread.php?p=1773584
    MergedFB: http://www.ubuntuforums.org/showthread.php?p=1773710
    BigDesktop: http://www.ubuntuforums.org/showthread.php?p=1773544

thanks to www.ubuntuforum.org for this valuable article... Good luck guys !