It sounds like this has been brought up many times before, but this is almost a show stopper for us.
We really need a way to mirror subitems - or be able to put a board in two locations at once as a whole.
I was busy mirroring boards and then realized the subitems were not there! What use is it, then?
Agreed! This would be absolutely crucial for our workflow. Please add this feature! 
+1 can we please get this? The current feature with mirroring subitems with other tasks is kind of useless. Would like to be able to mirror subitems across boards instead
This would be a game changer for us. Linking master items to sub items across different boards is critical.
Agreed! This would be highly beneficial to my team’s workflow as well.