In multi monitors setups, Xubuntu / Xfce clones (mirrors) instead of expanding the display to the external monitor. So here are two ways of getting proper multi monitor support in Xfce / Xubuntu.
I. Xubuntu 13.04, 12.10 or 12.04 only: use the Xfce 4.12 PPA to get proper multi-monitor support
Xfce 4.11 (Xubuntu 13.04 and 12.10 use Xfce 4.10 by default) has implemented support for extended desktop mode, so using the Xfce 4.12 PPA in Xubuntu 13.04, 12.10 or 12.04 (this is a development PPA which provides some development packages), you'll get proper multi-monitor support by default.
Using this PPA, in the Xfce Settings Manager, under "Display", you can select the position of each monitor (left of, right of, below, above or the same), and if you want to mirror the displays or not:
To add the PPA and upgrade these packages in Xubuntu 12.10 or 12.04, use the commands below:
sudo add-apt-repository ppa:xubuntu-dev/xfce-4.12 sudo apt-get update sudo apt-get dist-upgrade
Once the packages have been successfully upgraded, open the Settins Manager, and under "Display", uncheck the "Mirror displays" option to extend your display to the external monitor(s), then select the displays position, etc.
There's also a minimal display dialog (thanks to Simon Steinbeiss for the tip!) you can use once you've added the above PPA, in case you need to quickly change monitor settings, which you can run using the following command (you can create a shortcut for it if you often change your monitor layout, .e.g. when using a laptop with external monitors):
Here's what you'll get:
II. Any Xubuntu / Xfce version: Use Arandr to extend the display instead of cloning (mirror) it
1. Install Arandr, a simple GUI for XRandR:
sudo apt-get install arandr
2. Then, open Arandr and move the monitors to the desired position using drag and drop:
By default, the monitors might be displayed one above the other, so drag the first one to be able to see both of them.
When you're done, select Layout > Apply.
3. The changes are not saved and will be lost once you logout / restart the computer so to make them permanent, from the Arandr menu select Layout > Save As, enter a name for this layout and save it.
This creates a script under ~/.screenlayout using the name you've used above (unless you've selected a different path).
4. Add this script to your startup items: Session and Startup > Applications Autostart - click "Add", under "Name" enter whatever you want and under "Command", enter the exact path to the script created under step 3 and you're done:
Your Xubuntu computer's display should now be extended to the external monitor(s), instead of using the default clone (mirror) option, each time you log in.