I ended up with using the data from the $_POST on the OnDocFormSave event and then update the database if needed. I made a plugin called AutomaticLongtitle the code is pasted below. Remember to register the OnDocFormSave for this plugin if you decide to use it.
$e = &$modx->Event;
// Only on this event
if ($e->name == 'OnDocFormSave')
{
// Check if the longtitle is empty
if ($_POST['longtitle'] == '' && $_POST['pagetitle'] != '')
{
$fields = array('longtitle' => $_POST['pagetitle']);
$table = $modx->getFullTableName('site_content');
// Update the database
$rows_affected = $modx->db->update(
$fields,
$table,
"id = {$e->params['id']}"
);
// Generate an error if the DB was not updated
if(!$rows_affected)
{
$modx->event->alert("AutomaticLongtitle: An error occured when saving the longtitle.");
}
}
}