Split menu won't show on all pages

The problem:

We have the main menu (top / parent level) on "all" pages.

We wanted the second level menu to appear on some pages that were not directly part of the menu.

So for example we have a page called "about Joomla!" - there is no main menu item or sub-item pointing to this article.

We have implemented a hidden menu system & it contains a menu item pointing to the "about Joomla!". However, when we selected this item to display the split module - nothing happened.

Our solution:

We created a menu item linking to "about Joomla!" on the menu that we were splitting - in our case the main menu - and then selected that menu item when choosing to display the second level split menu.