We do this all of the time. It is not uncommon in the
DVC realm as you use points from different contracts for the same stay... Just tell them at check in that you have a second reservation and would like to stay in the room. As said you do have to check in twice and they will issue you new keys... Just as a warning...if you use your room key to lock the room safe make sure to unlock it before you check in for the second leg of your stay. I didn't and when i checked in the second time the desk CM took the old keys and gave us new ones... Meaning i could not get into the safe...