Howto “split” the page list into top level menu and separate submenu?

Hi everyone, I want to do the following but don’t know how:

I want a horizontal bar menu showing only the top level page items… This is quite common now on modern websites and can be easily done with WordPress.

But then I want a separate, folding menu which only shows the subpages (tree) of the currently selected top level item. I do not want the other top level items showing up in this menu.

In other words, I need a wswwpx_fold_page_list that automatically skips items that do not have a parent.

Can this be done in the template using wswwpx_fold_page_list, assuming I have already written a large tree of pages…

Please give me some hints. I’m willing to experiment and try on my own if you give me some clues. Thank you.