We just spoke with DCL a few minutes ago and asked this exact question. No, you cannot book multiple cruises with the credit. You can only apply it to one booking and the balance will be converted to an OBC. Anything leftover will be lost.
So, if you don't spend the OBC amount while on the ship you lose the rest? In other words you don't want to score a GREAT deal unless you plan spend a couple thousand on board?