Since Ubuntu 14.04 was first introduced there has been a known bug where on some systems - the ability to run multiple monitors is not completely possible. If you are trying to run multiple monitors on Peach OSI and you are experiencing problems please refer to the following URLs in order to hopefully help you get your multiple monitor setup up and running. Since this bug does not effect all systems there isn't much that we here at Peach OSI can do to help your situation until Ubuntu decides that this is a big enough issue to put forth a permanent fix.
http://askubuntu.com/questions/450767/multi-display-issue-with-ubuntu-gnome-14-04and
https://bugs.launchpad.net/ubuntu/+source/gnome-shell/+bug/1292398