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!