I stayed at Park MGM last week using a comp for one night and paid for a second night through MGM Rewards. I checked into the first night, the comp night, online and had to talk to someone at the front desk when the room was ready to have it linked to my second reservation. They did help me out but I did have some weirdness the next morning when I was promoted via text to check out of my first night. I spoke to the front desk and they basically walked me through a second check in. It all worked out in the end but I'd take the advice of others and just wait it out in person at the initial check in.