How to nest form into another form?
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;
}