you have to edit the xorg.conf file by hand
goto
http://www.tldp.org and look up multiple monitors or dual monitors, they will have a tutorial for it im sure
basically you have to add the extra lines for the new hardware similar to the one thats already there, plus a couple options dealing with position---right,left, center(if youve got 3)
better have a look at the X man pages too, this gets a little complicated to setup, its not real hard, but you will need to know what to put in the xorg.conf file, its well commented so that helps
I think you have to change the display environment variable(s) too, not sure on that, cant remember if its gonna treat them as separate displays or independent, I think that is one of the options in X.