I have finally managed to use twinview with a TFT(1440x900@60Hz) and a TV(PAL,1024x576@50Hz) on nvidia (proprietary driver), configuring xorg.conf manually. I had to do this manually since nvidia-setup tool always mixed up the two resolutions and capabilities of the different displays resulting in X not starting at all, and also switched primary display to the tv, which I didn't like very much.
Moreover this tool from nvidia isn't able to cope with the different settings of TV-OUT (PAL, 50Hz, COMPOSITE, S-VIDEO ...) their card and driver needs to hand out the correct signal. There were some lines which only can be set manually, after all. I can't make use of two seperate x-screens combined by xinerama since I use compiz on the desktop.
Now everything is fine, except for the screensaver and energy-management: If I activate screensaver or energy-management to blank the TFT while displaying a Video on the TV-Out, it will blank both displays instead of leaving the TV-out play along while the TFT powers down only.
How can I tell X to leave the tv-out untouched while TFT is switched off or screensaver blanks it? I have noticed I have to move the mouse on every display seperatly to wake them one after the other, so I think they are handled independent from each other, so I hope it is possible to configure them seperately.