I want to keep showing “submenu” on the left

I want to make a website with a horizontal menu and a vertical (left) menu. When clicking on an item in the horizontal menu in the left menu the items should be shown under SUBPAGES1.

Example:
You can see this here http://69.89.31.249/~lustenla/?page_id=16, when clicking on HOOFDRURBIEK 2 (on top). In the left menu you then see “hoofdrubriek 2 – sub” and “hoofdrubriek 2 2 – sub”. This is ok.

But…when clicking on “hoofdrubriek 2 – sub”, you get to see the new page, but, the SUBPAGES1 menu on the left side is gone. I want the submenu items to be displayed, still.

I suppose I can change something in the SIDEBAR.PHP file (but I dont know how), but maybe there is another sollution or maybe, there is a THEME that does what I want. This does not seem to be such an unusaul request after all.