How do I setup dual monitors in XFCE?

First, open up monitor config - it's in Start > Settings > Settings Manger, then open the Display item.

Make sure both your displays are on.

Then, open a terminal and run this:

xrandr

The output will look something like this:

Screen 0: minimum 320 x 200, current 2464 x 900, maximum 4096 x 4096
LVDS1 connected 1024x600+1440+0 (normal left inverted right x axis y axis) 220mm x 129mm
   1024x600       60.0*+   65.0  
   800x600        60.3     56.2  
   640x480        59.9  
VGA1 connected 1440x900+0+0 (normal left inverted right x axis y axis) 408mm x 255mm
   1440x900       59.9*+   75.0  
   1280x1024      75.0     60.0  
   1280x960       60.0  
   1280x800       74.9     59.8  
   1152x864       75.0  
   1024x768       75.1     70.1     60.0  
   832x624        74.6  
   800x600        72.2     75.0     60.3     56.2  
   640x480        72.8     75.0     66.7     60.0  
   720x400        70.1

Then, run the following, changing VGA1 and LVDS1 to match the appropriate display:

xrandr --output VGA1 --left-of LVDS1

Note that you can move change --left-of to --right-of.

Now, it should work, but you've still got one problem.

It will disappear after you logout. So, you need to add it to your login items.

Head over to Start > Settings > Settings Manger, then open "Session and Startup", add the above command to your login items, and you're good to go!

There is a different solution to this problem.

According to this article, xfce 4.11 and 4.12 have implemented multi-monitor set-up out of the box. I guess this question will therefore be obsolete for ubuntu 13.04.

Meanwhile, for ubuntu 12.04 and 12.10, one can use the xfce 4.12 ppa :

sudo add-apt-repository ppa:xubuntu-dev/xfce-4.12
sudo apt-get update
sudo apt-get upgrade
xfce4-display-settings -m

Now you can use Arandr.

It allows you to do that in visual editor and then save settings as shell script with xrandr command script.

Then you can add it to autostart.