Your question is good, however now what I have the method is firstly set your level one and level two with some identify,for example, pre- or what to distinguish, then use filter function to choose the level what you want.
I test as next,Add one button, Button.OnSelect:ClearCollect(test,Choices('20190418Metadata'.Country_City))
Then ComboBox.Items:Filter(test, "City" in Label)
Result Check: The Result will only show the city level.
This is what I have thought , maybe there is one better solution, if you find one more perfert, welcome update the post.