place dropdown and listview inside the dataview, set DS_microflow for the listview, pass the dataview object into DS_microflow based on that dropdown value you can retrive list in DS_microflow.
I think you are looking for the Constrained by functionality. See the documentation here: https://docs.mendix.com/refguide/reference-selector
This way you you can have two dropdowns where the second wil show values dependent of the selection of the first dropdown.
If you you want show the second drop down on the selection of first drop down item in the same data widget, you can use conditional visibility option. Just right click on second dropdown and set the radio option on conditional visibility. Put the condition and run your app.. Hope it will work. If so please give feedback.