Thank you for your response,
the reason for doing it as described is, that, if i want to watch a movie, i want to watch it on my TV. But I don't need two screens switched on. If I want to do something on my computer I want to return to my normal screen vt7, and to continue watching the movie on vt8.
Now I solved the problem (by chance): In the tvout script (as posted above) I replaced 'X -screen TV :1' by 'XFree86 -screen TV :1' and now everything is working. Anyone could tell me why?
P.S The special thing with this configuration is, that you can replace mplayer by every other program and so e.g. play games on your TV. And if your crashing the X-Server nothing happens to your current (working) X-Session.