You can book a cruise on points 11 months ahead of departure.
I'm not quite sure what you mean when you ask "Can I book one room on points and another with cash?" As far as I know, you cannot book a cabin through MS entirely with cash. At least one person (adult) per cabin has to be paid for with points.
Since you get your cabin assignment when you book, you might be able to book one cabin with points via MS and if the adjacent cabin is still available, book it for cash via the "normal" way. (If that is what you want to do). Good Luck!