I booked two nights separately recently as it was cheaper - ended up having to move rooms as got an upgrade on the 1st night but hotel was full 2nd night so had to move to the standard room!

I did email the hotel to ask to be in the same room but they didn't bother reading the email!

I have done the same in another hotel and we had the same room as they were not full.

I would ring the hotel and see if they can book you in to the same room - you might have to check in each day, but they may just keep shifting the charges to the final night stay.