I believe I ran into a very similar problem on my site. The client requested two distinct menus, one for the very top of the page, and one for them middle of the page. The menus needed to both be dynamically updated as pages were added/modified on the site, but they couldn't share any content. To solve this problem, I also created the site hierarchy around the menus. It looked similar to this:
Root > TopMenu > Home
> About Us
> SubMenu > Events
> Get Involved
Similarly, the client had a fit when they saw that the URLs were generating site.com/topmenu/home.aspx, site.com/submenu/jobs.aspx, etc.
The simplest solution that I found was URL Rewriting, and if you happen to be using IIS7, then you're in luck, because if you download and install the URL Rewrite Module, then you're in business. Check out this page for installation info: http://learn.iis.net/page.aspx/460/using-url-rewrite-module/
If you're using IIS6, then I'm not 100% sure how to accomplish this, but I bet this resource is a great place to start. http://www.simple-talk.com/dotnet/asp.net/a-complete-url-rewriting-solution-for-asp.net-2.0/
I hope this helps!