NJDave- I think Marriott's initial intent for the 13 month rule was for use and not exchange, so initially I remember reading that generally only one reservation number was given unless you specifically asked for two. It appears that they just routinely give separate numbers and, as Sue stated, just ignore the reality that many if not most of the units booked that way are being booked to grab harder to get weeks for trading or renting purposes.
It actually surprises me that they don't enforce it, since at some of the resorts with too long a Platinum season getting a summer week, for example, can be a real issue. At places like NCV, for ex., I would assume without people booking using the 13 month rule and then splitting the weeks for either deposit or rental it would be easier to get a summer week.