Michael the easy way to do that is by using a super powerful yet often under stated and under employed feature of WordPress using TPL files.
When I build custom templates for anyone I often build a few features into them like custom error.header.php file for the 404 page and then depending on the need I'll build several template files named like.
If looking at http://www.lookforcoloradohomes.com we are using WordPress and our real estate plugin to do what you are seeking. The site owner will have a menu titled "Sub Divisions" for example with menu links to those WordPress pages, when the visitor lands on those pages using an assigned template they then get links on the second level for links associated to that sub division.
Method two would be to code your menu using jQuery or MooTools with an expanding menu that shows the sublinks when the parent link is ACTIVE. I have code and I will publish it by this Friday on the support site because people frequently ask about this.
If you are in a hurry please feel free to contact me via the support site at UltimateIDX.