Marriott has no paper certificates for most awards. Therefore, I think it would be allowed since reservations can have a Guest #2 listed and authorized to check-in the reservation. The person checking in needs to present their own credit card for incidentals.

I know I've had other people check-in my Marriott stays. (Granted, I'm always showing up later that day or perhaps the next day, but the front desk never knows or cares.)