Show terms from a taxonomy only if another taxonomy is present

Here is a general overview of how my blog is structured

Site Structure

1. Lists out all posts unfiltered
2. At the top of the homepage there are 3 links for different “Business Units” which is a custom taxonomy. If a user clicks on one of these business units then it will show the archive page that filters for that specific business unit.
3. A Sidebar that has filters for a custom taxonomy of “topics” and taxonomy of “tags”. While on the homepage if a user clicks on a certain topic or tag it will show topics or tags for all “business unit” which is what i want.

Post Taxonomy Structure

1. Tags
2. Topics
3. BU

My Problem

1. I want if the user clicks on a specific business unit the sidebar will change so that it will only show topics or tags that will filter for just that specific business unit rather then showing all posts that have that taxonomy.

Example Scenario

1. User comes to the home page and clicks on Business Unit A link
2. They are taken to the Business Unit A archive page, which displays all posts that belong to Business Unit A.
3. User then clicks on a specific topic “Meetings” in the sidebar while on Business Unit A archive page
4. Posts displayed are of bu=”A” and topic=”Meetings”