I had the same problem, and I just managed to fix it.
In a terminal, type:
sudo gedit /etc/X11/xorg.conf
You should see this section in the file:
Section "ServerLayout"
Identifier "aticonfig Layout"
Screen 0 "aticonfig-Screen[0]-0" 0 0
Screen "aticonfig-Screen[1]-0" RightOf "aticonfig-Screen[0]-0"
Screen "aticonfig-Screen[2]-0" RightOf "aticonfig-Screen[1]-0"
Screen "aticonfig-Screen[3]-0" RightOf "aticonfig-Screen[2]-0"
EndSection
You need to move the second screen beyond the boundaries of the first, and the remaining two will follow it. In my case, 2000 pixels is enough since my resolution is less than that in both directions; adjust for your monitor as needed.
Section "ServerLayout"
Identifier "aticonfig Layout"
Screen 0 "aticonfig-Screen[0]-0" 0 0
Screen "aticonfig-Screen[1]-0" 2000 2000
Screen "aticonfig-Screen[2]-0" RightOf "aticonfig-Screen[1]-0"
Screen "aticonfig-Screen[3]-0" RightOf "aticonfig-Screen[2]-0"
EndSection
Restart and you should be all set.
Let me know if this worked for you!