so now i had some time to take a 2nd look at the xorg.conf file so this is my try. I added the following to the /etc/X11/xorg.conf (added it to the already existing entry which i thought was for my laptop's monitor )
the settings should be properly. Read them up in my laptop's datasheat.
Identifier "My External Monitor"
HorizSync 31.5 - 150.5
I added this part in order to enable both monitors (both work but only one at a time ) so i thought to add a 2nd line for a second device. startx after this but nothing happened. I also followed your tutorial on how to create a xorg.conf file but this didn't help much either (I didn't found anything concerning dual monitors in the conf builder).