Thanks to BerndJM in the german forum I got a solution: If you use show_menu2 and modify the menu call like this:
<?php you can put as many <br /> in the menu title as you want. Only negative aspect: the filenames will look a little weird then.
show_menu2(1, SM2_ROOT, SM2_ROOT, SM2_CURR|SM2_PRETTY);
$menu = ob_get_contents();
$menu = str_replace('<br />', '<br />', $menu);