How can I add a second submission handler to a form to run after the default handler node_form_submit
All the methods you have mentioned should be calling your custom submit handler at the end. However check your $form array when the page is loaded to confirm your custom submit handler is at the right place.

If not try 

MYMODULE_entity_insert($entity, $type) {

//Paste your secondary node creation code here

}


See https://api.drupal.org/api/drupal/modules%21system%21system.api.php/function/hook_entity_insert/7 for more details on this hook