Handling Complex Navigation

To set this up: I am developing a magazine-type site with 8 main categories. Each category will have a few subcategories. Certain categories like Restaurants or Travel will be paired with a set list of Major Cities.

In the category archive view, I would like to list the main category’s subcategories and then a separate list of Cityname (travel posts in this city) i.e.: Los Angeles (4)

I may be over-complicating this but is there code that will let me take one category, compare it with a city category and then return the amount of posts that it contains?