I've combined a paid reservation and a "free night certificate" or points reservation a number of times.

I make sure I've booked exactly the same room category (same view, same number of beds, same designation such as "deluxe" or whatever the hotel calls its standard rooms). Sometimes I cross-reference the reservation number in the comments, requesting to stay in the same room.

With both confirmation numbers at hand, I call Marriott Reservations to link/notate the two reservations.

At check-in, I verify that I'll be in the same room. It's never been a problem. Usually, I don't even have to go to front desk on the day of the switch-over, and the same key cards continue to work.

I'm not saying anyone on this thread gave bad advice about tracking down someone at the hotel. I've just never found it to be necessary. The request just needs to be clear to the hotel's room assigner. Exactly how the room assigner gets that message is less important.