OK, I solved the issue. To explain:
It was from this manual (I was getting the same error in Gnome/Display settings):
http://askubuntu.com/questions/126392/problem-connecting-multiple-monitors-on-12-04-problema-al-conectar-monitores-ubBut creating virtual area and the xorg.conf file seems to have solved the issue.
Virtual 4096 4096 to the Display Subsection of my /etc/X11/xorg.conf: SubSection "Display" Virtual 4096 4096 Viewport 0 0 Depth 24 EndSubSection Kind regards, Jan