Did you at some point in the past use "Switch User" to start a new
session on tty8? It would be interesting to know what options are used
when the second Xorg process is started. Here is what I see:
$ pstree -a $(pgrep -P1 kdm)
kdm -config /var/run/kdm/kdmrc
├─Xorg -br -nolisten tcp :0 vt7 -auth /var/run/xauth/XXXXXXXXXX
└─kdm
└─startkde /usr/bin/startkde
├─kwrapper ksmserver
└─ssh-agent /usr/bin/dbus-launch --exit-with-session /usr/bin/startkde
This should tell you on which VT the second Xorg process is running; you
can then switch to that VT and check out what is going on. (I think it
should never happen that two Xorg processes are attached to the same
VT; lockfiles in /tmp are supposed to prevent that.)