How to nest form into another form? I have 2 modules. Let's call them Publisher and Publication for discussion's sake.

Publication module requires Publisher module. Publisher module provides form to edit publisher's data. Publication module provides list of publications with reordering and adding capability, and it can be limited to display only publications by one publisher.

Now, I want to nest publisher's publications form at the bottom of his data edit form. I would prefer to avoid writing same code again - I want to keep validation functions and submit functions as they are now, just have it on one screen, with one submit button.

Subform seems to be unmaintained and buggy at this point and Multiple forms seems to skip validations altogether, that makes both of them unusable for me.
You can use the admin/content page as a reference to combine forms. There, the filter form and overview form are passed as elements in a new $form array. 
Coming from the content administration callback function in the node.admin.inc file:


/**
 * Menu callback: content administration.
 */
function node_admin_content($form, $form_state) {

  $form['filter'] = node_filter_form();
  $form['#submit'][] = 'node_filter_form_submit';
  $form['admin'] = node_admin_nodes();

  return $form;
}