No, I think you are absolutely onto something -- as you describe it, it is trivial to transform from N -> 2N districts in a manner which makes it impossible to

No, I think you are absolutely onto something -- as you describe it, it is trivial to transform from N -> 2N districts in a manner which makes it impossible to combine them back 2N -> N in any other way than the original map. Maybe I'm missing something too. That seems like a pretty big deficiency.