Unfortunately, I do not have very good luck with the Fedora Core display configuration panel, however it is easy enough to set it up via editing /etc/xorg.conf.
If you are using and ATI or an Nvidia graphics card, hopefully I can point you to the right direction.
Follow the instructions on
http://www.fedorafaq.org/
in order to install the ati or nvidia graphics drivers. You will need to install the livna repo (directions also in the fedorafaq).
With nvidia based cards you need to use twinview for dual spanning display that comes with the drivers, all I do to enable spanning with nvidia in FC4 is add the following to the device section in xorg.conf. Yours might not look exactly alike however this is a starting point.
Section "Device"
Identifier "Videocard0"
Driver "nvidia"
#make sure you specify nvidia instead of nv(nv is the generic driver, nvidia is the one from the livna repo)
VendorName "Videocard vendor"
#twin view settings below
BoardName "nvidia"
Option "TwinView" "yes"
Option "MetaModes" "1024x768,1024x768;800x600,800x600"
Option "ConnectedMonitor" "CRT-0, CRT-1"
Option "TwinViewOrientation" "CRT-0 LeftOf CRT-1"
EndSection
The device, vendorname, and boardname should be different on your setup, I just leave the default names there since I have no need to change them.
For ATI based cards there is something called Xinerama that configures dual spanning display, however on my laptop, I do not have to mess with that, all I have to do is add the following under the device section
Driver "fglrx" #driver from livna repo (fedorafaq.com instructions)
Option "DesktopSetup" "Horizontal" #for some reason all I do is add this to the driver section, press ctrl-alt-backspace to restart X and dual display works using an ATI Xpress 200m
If you have an ati card and this does not work, I would do a google search for Xinerama and xorg.conf
Hopefully you are using an ati or nvidia based card and that this will help you.