If this worked you will probably want to change your X configuration to start X this way automatically. There are lots of ways to do this and there are probably better ways than what I've shown below. Here are the methods I used for changing startx and gdm.
If you run X from the command prompt add the +xinerama option to the startx script.
root# > vi /usr/X11R6/bin/startx
At the very end of the file the script starts X with the command xinit. Add the +xinerama option to the very end of that line.
To Configure gdm to start with the xinerama extentions look for [servers] section at the bottom of /etc/X11/gdm/gdm.conf. Add the +xinerama extension to the end of the line that starts with 0=. It should look like this :
0=/usr/bin/X11/X +xinerama
I am certain configuring KDE, and xdm to start xinerama are equally easy, if you figure it out please drop me a note and I will add it to this Howto.