If the regular visibility based on expression is not working, you could add a visibility flag to your object in the dataview. Show the second dropdown based on that flag.
If the first dropdown changes, execute an OnChange microflow/nanoflow that recalculates the visibility flag and refreshes the data view object. This will toglle the visibility on the second dropdown.
Are the drop downs enumerations? Or are they associated entities? If you are using enumerations, this is straightforward:
If you are using associated entities, you’ll need to select ‘based on expression’ and create an expression to control visibility.
Hope that helps,
If you are using reference selectors you could trying using Constrained By. There’s an example of how to implement in the Mendix docs for two dropdowns.