Found a solution which is working.
Splitting the array like above just gives you the plain text so shortcodes wont work.
this works:
global $post;
$content = get_extended( $post->post_content ); //get the_content
$excerpt = $content['main']; //get the teaser part before more tag
$main_content = apply_filters('the_content', $content['extended']); //get part after more tag
echo $excerpt; //post teaser part
echo $main_content; //post main part
my head is smoking. too much for a beginner
and if you want to have
in your html use this:
echo wpautop( $main_content );