I'm not an expert by any means, but I think you should try to get this taken care of before you go. On another thread, someone explained that room assignments are done by schedulers a week or so in advance, not by the person at the front desk. It's a very complicated thing getting all those rooms filled for the right length of time without leaving orphaned rooms. Even if you had the same room category for both bookings, there's no guarantee you'd get to keep one room for the duration of the split stay. If you wait until arrival, there's a very good chance there won't be anything available in either category for the whole length of stay.