The only way to do this is to book it as different reservations. The problem with this (unless you are an AP holder and book one part as an AP ticketless package), is that you have to have tickets for each one.
So, the savings for changing plans is usually eaten up by the extra money for tickets (although those can be saved for future use). And, when you have two different reservations, you do have to check out/in and can ask to stay in the same room, but I don't think they can guarantee you that you can.