Create a web form with two radio buttons in the same group. Call them 'First' and 'Second'.
Clicking First displays an additional dropdown menu called 'Menu' populated with 'Item_First_One' and 'Item_First_Two'.
Clicking Second displays an additional dropdown menu called 'Menu' (same name as before) populated with 'Item_Second_One' and 'Item_Second_Two'. The menu previously displayed when First was clicked disappears.
If First is clicked again, then its menu reappears and the menu for Second disappears. The show/hide status of the two menus therefore relate to the state of the radio buttons.
I don't need the form to do anything. I just need to see how it is done then I can adapt it into my site.
Clearly, the selections in the menus must be capable of being passed by the form when a Submit button is clicked.