Linking two vocabularies

Hi everyone,

I'm new to taxonomies and am looking for some advice on how i can achieve the following functionality:

We're building a business directory site, and users will be able to post their own business listings. There will be a set of top-level categories such as "Construction", "Restaurants" etc. which is a fixed vocabulary predefined by us. When a business adds a listing, they must choose one or more of these top-level categories. They must also enter one or more of their own subcategories via free-tagging. So a business might choose "Construction" as their top level category and enter "Widget Supplies" as their subcategory.

We also would like the subcategories entered by a business to be linked to the selected parent top-level category(s). This is important because when a user selects a top-level category such as "Construction" we want to display as list of all the subcategories that were entered by the various businesses within that top-level category:

Construction
--> Widget Supplies
--> Equipment Hire
...

I'm not sure how to achieve this because the it seems that we need two seperate vocabularies; one for the top-level categories, and one (or many?) free-tagging vocab(s) for the subcategories. How do we link the two types of vocabulary so that each set of subcategories has a parent top-level category?

Thanks,
Peter