FileMaker does not have a built in tool for creating such a layout.
Closest, simple way to do what you want would be to create two layouts--one that lists only the Branch offices and one that includes the department names. A button can switch you back and forth between layouts.
But this will "expand" and "contract" all of the department names not just the names for one branch office.
You could also set up a button from a list of branch offices that pops up the department list in a window that floats in front of your main window.