We had 2 separate booking when we sailed on the Discovery. Both insides, on different decks. We just asked onboard that we each be allowed access to both cabins. It was done immediately with no questions asked.
we only put our stuff in one cabin, and had no issue with the cabin steward. We did, out of courtesy, let the other steward know we would not be using the cabin, but checked in when the immigration and disembarkation documents were delivered.
we saved a little, but more importantly for us, got 2 cruise credits each, one step closer to elite!