OP, as others have said, you definitely can use two DSUs at the same property. I have done it as well. However, one thing you will have to be mindful of, it is at the discretion of the hotel to give you Diamond benefits for both rooms, e.g. breakfast/lounge access. When I did the upgrade, I was given access to the lounge for both rooms though. It just depends on the hotel. I think many are friendly in that but it should necessarily be expected.