Hi,
Certainly too late, but here's some code that should do the job. All the fields, including 'art_ttitle' must be in the form to be available in the $fields array :
// Set title value
$title =$fields['textauswahl']->value . '-' . $fields['datum']->value;
//Affect title value to 'art_title' field
JCckDevField::updateValue( 'beforeStore', 'art_title', $title, $fields, $config );