We've got a instance where currently people are ordering posters and a book. As it's one order it is a pain for packing as the poster must go in a poster tube and the other in a bookwrap. It would be ideal if we could tell the sku at the product level to always be sent on its own as it has to go via a different method.
It already exists, you need the order rules module. It allows you to create rules to split an order based on the product category.