For anyone finding this (like I did) while trying to solve this problem today, this is the best solution I could come up with:

Cascading Parameter Example | Tableau Public

The basic mechanics (take a look at the public workbook above to see it in action):

• Separate sub_parameters for each main_parameter option

• All sub_parameters are floating and stacked on top of each other on the dashboard

• Visibility of sub_parameters is controlled with the "Control visibility using value" setting on the Layout tab. This points to a separate calculated boolean field for each sub_parameter so that only the appropriate one is showing at any given time

• A final calculated field chooses the correct sub_parameter based on the main_parameter selection.