In order for the "Switch Session" option to work properly, you need to be using the KDM instead of the GDM as your display manager. Edit your /etc/sysconfig/desktop
file to look like (the uncommented lines in) this:
# cat /etc/sysconfig/desktop
Once you do so, the KDE menu will have a "Switch Session" entry which you may use to start a new session using any valid user id and session type you've configured your system to have available. It's very handy when you've got something running and someone else needs to log in to the system for some task, or when you want to check something in GNOME when you're in KDE. Note:
While you can switch between sessions from the KDE menu, GNOME does not support such switching, so you need to use the Ctl-Alt-F[7-0] (or logout) to switch out of GNOME.
The reason that you have a blank screen after Ctr-Alt-F8 is, of course, because you have not started an X-session on that (logical) screen, and with no X-server processing the screen, nothing will happen when that screen is connected as the primary display. What the KDM manager does is let you configure the X-server to handle more than one screen.