start new session and get kdm login screen

Hi all, I want to be able to start a second X session out of my current session. I know this question has been raised several times, and the start itself is not my problem. I have multiple entries in the Xservers files, and the second session comes up. No problem. But I am already logged in as the user under which I started the session. What I want is to start the new session as root (e.g. via some SUID program), and want the kdm login screen to appear so that I can choose which user and session type I want to start the session under. And I want to be able to do this out of any session type I am currently running (kde, gnome, xfce...) Any idea how to achieve that? Thanks in advance Mario