Honestly, your best bet is to book and apply the DSU with whatever other payment you are using for the other nights...points, cash etc, then after applying it, book the single certificate night after. Unless you are trying to book 1 night only with a cert and use a DSU for that one night. I have never once encountered a hotel that has made me move for that 1 night, despite being fully entitled to (and I am always prepared for that should they wish to adhere to the rules and their right to move me).