The outputs you display above only report the existence of one monitor, not two. If you have given the above commands when you had both external monitors connected to your pc, then most likely there is a driver problem. You will have to investigate this by sifting thru the error messages in /var/log/Xorg*.
The reason why I am saying this is that X is, by now, essentially self-configuring, so if anything fails, it is most likely due to the absence of some essential component.
You may at any instant try to allow X to reconfigure itself by means of
sudo dpkg-reconfigure xorg Alternatively, you may follow the ever helpful Arch Linux wiki about MultiHead (which is the configuration you are attempting). But do not be confused, all Linux distros use X, and are thus strictly equivalent in coping with your configuration.