I have a set of dual dynamic headphones in my cupboard - the old Akg playbacks. That was a pointless gimmick. Both drivers are sharing the same jobs, enclosure front and back. That's not what I find impressive. This was the first time I have seen a dual dynamic iem, with the "tweeter" enclosure creating a kind of acoustic transformer for the "woofer". If you know of another, let me know the model.
Here let me google that for you