Per Marriott's T&Cs one person is supposed to receive points for upto 3 rooms per night, but will only receive one night/stay credit per night. The pertinent rules are Marriott Rewards Earnings T&Cs.

With that said, at non-FS hotels, most of the times I have booked more than one room, I have received night/stay credit for upto 3 rooms (and points for those rooms as well.) At FS hotels, most of the times I have booked more than one room, I will receive only one night/stay credit and correctly receive points for up to 3 rooms. (I think FS and non-FS hotels have a different system they use which is the reason why one follows the rules more closely than the other.)

So the moral of the story is don't bank on getting more than 1 night/stay credit per night even with 2 or more rooms. You might get lucky and receive more night credits, but you might not. If it were me, even though I have received extra night credits per night at non-FS hotels, I still wouldn't gamble with my money on it working. I'd just go ahead and get one room at a time.