module_invoke to create user search block not working
The search block only works for the default search type. From search_box_form_submit():

$info = search_get_default_module_info();
if ($info) {
  $form_state['redirect'] = 'search/' . $info['path'] . '/' . trim($form_state['values'][$form_id]);
}
else {
  form_set_error(NULL, t('Search is currently disabled.'), 'error');
}


The third parameters you're passing ('content_search', 'user_search') are ignored. Presumably your default search type is content and that's why you're being redirected to the content search in both forms.

You can build the correct search form pretty easily though:

$content_form = drupal_get_form('search_form', NULL, '', 'node');
$user_form = drupal_get_form('search_form', NULL, '', 'user');