it was easy in fact, even if getting field value is easier after the save of the post 🙂
function argu19_set_ending_date_if_empty( $post_id ) {
if ( !isset($_POST['acf']['field_5bd0266e9f955']) ) :
$value = get_field('event_startingdate');
update_field('event_endingdate', $value);
endif;
}
// Apply to all fields.
add_filter('acf/save_post', 'argu19_set_ending_date_if_empty', 10, 3);