Give this a try. Edit /etc/X11/xdm/Xservers
Change this line from this.
:0 local /usr/bin/X
To this.
:0 local /usr/bin/X :0 vt7
:1 local /usr/bin/X :1 vt8
Drop to runlevel 3 with the command ' init 3 ' and back to runlevel 5 ' init 5 '
Found the above from here
http://wiki.linuxquestions.org/wiki/...Single_Monitor from a quick google search. Many more searches found as well.
Lets us know how it worked.
Brian