I have a GeForce connected to the monitor and to the TV. I want the TV only to play videos so I don't want any window manager in it.
By default with Mandrake KDE starts on both screens, and I need help to make it only start in monitor.
X is configured to use a single session with two screens, :0.0 and :0.1.
Looking at startkde script there are two interesting lines:

Looking at help, ksmserver supports "--display" but kdeinit doesn't.
And there are two interesting environtment variables but that I don't know exactly what they do and where are set:
KDE_DISPLAY
KDE_MULTIHEAD
After starting KDE, KDE_MULTIHEAD is set to true and KDE_DISPLAY isn't set. KDE_DISPLAY perhaps is no used anymore on KDE 3.x....but not sure.

I have tried a lot of options with different results, but not a good one. Setting the --display for ksmserver seems to partially work, there are some "kxxx" that are only executed one time, but others like kicker are still executed two times....probably because kdeinit has not specified anywhere the display screen.

Well...all this isn't of much help but if someone has experience with this sitaution or has any idea would be apreciated.

And anyway, someone knows if screen :0.1 can be shutdown temporally in a way I can use the main monitor normally, and activate the second (TV) screen only when I want to watch a video?

i don't know if this will help or not but the aswer might be in startx/xinit instead of startkde/kdeinit
i think x needs something running on the display for it to stay up
that's why when you close a window manager started with xinit x dies
try somethig like
exec startkde -display 127.0.0.1:0
exec something_else -display 127.0.0.1:1

or something like that
just thinking out loud and have never tried this or even used a dual displayhead setup