I don't think this is possible. As you said you can copy and paste drawing sheets from one to the other but this isn't providing any sort of link between the two. I guess it's the same as the difference between a derived sketch and a copied sketch. You're looking for a derived drawing sheet which unfortunately doesn't look like it exists. It could be a nice feature but generally it's either all within the one drawing or detailed separately. If you were going to combine them it would have to be when you are releasing them etc.