Did you know you can set filters to cascade in Tableau? Cascading filters are when the selection on the first filter causes the second to only show values that are relevant. For example, if you connect to the Sample Super Store data-set and select the Category of "Furniture" in a filter, the subcategory filter will only show values that fall into the Furniture category. These filters are also called Hierarchal filters.
How do you apply this to your dashboard?
Right-click on your Sub-Category filter and select "Only relevant values" from the drop-down.
Now instead of displaying all of the Sub-categories in your Sub-category filter, your filter will only display the Sub-categories that fall into the Furniture Category. The Sub-category filter will be dependent upon the first or Category filter.