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 :
I have never set up kdm, or xdm for Xinerama, but I got the following tip from Dalibor "email@example.com".
Here's my changes to startup files for slackware 7.x (i use KDM and x4.01) edit /var/X11R6/lib/xdm/Xservers add +xinerama to the end of last line ie. :0 local /usr/X11R6/bin/X +xinerama It appears that KDM uses standard XFree xdm files, so this should work if you use xdm as well