Priceline will not qualify since you won't earn points on the room rate (although you can earn points for incidentals at FS properties if you add your account number to the reservation). You can earn stay credit only for one room if you reserve 2 or more rooms under one account (per Marriott's T&Cs), although you can get points for up to 3 rooms for one property on one stay. Some people have gotten night credit for multiple rooms applied for elite status, but I would be surprised if you can get the night credit for multiple rooms to apply to the promotion.