Called the SPG reservation line ( 888-625-4988 ) to book a Sheraton. First agent picked up, mentioned I wanted to use TWO Marriott Free Night Certs (25k points each, originally from Chase). They transferred me to another agent. They took my Marriott rewards number and details, asked which hotel I wanted to book and the dates. They made a reservation per free night certificate - for a two night stay, I have two separate reservations for the two nights at this Sheraton. I'm fine with this as the certificates were applied sucessfully!