PHP help with customizing the category menu widget

I am looking to customize the category menu widget to interact with a second similar menu. I want to use a couple categories that will only show in one menu and a different set of categories in another. The website is a small business directory. What I need to do is have the first menu show the list of counties served by the site. The second menu will show a list of products.

Posts will be dropped into two categories-product type and geographic county. The idea with the first menu is to change a variable in the second menu so that the output is show posts that are both in category a + b.

I hope this makes sense. Any help would be appreciated.