As Socrates states, you can earn points on the extra room, but not night/stay credits.

And you can actually get points for up to 3 rooms in a hotel per night (Marriott Points Earning T&Cs - check out bullet #1 under Earn Points.)

Now with that said, most every year, I grab a set of rooms for myself and family for a big family get together. This is usually at a non-FS hotel (RI, SHS, etc.). And I can't remember a time when I didn't get both points and night credit for 3 rooms. The times I have done this at a FS Marriott, I have always just received the points but not the night/stay credit. But as always YMMV.