it has been a while but I did it at the Delano in Vegas. I told the front desk at checkin so the room would be available for both days but, to be safe, I cleared myAmex bill the next day after breakfast to ensure Amex credits posted properly. I then checked in on my Chase reservation to the same room and asked for the keys to be reprogrammed. Next day I checked out on Chase reservation.

I got full credits for both reservations but YMMV depending on the desk agent that you draw.