Saw this promo when I went into my account to try registering for the Springhill promotion:

Marriott Rewards members who register for this promotion by September 30, 2011 will earn 5,000 bonus points with every second paid stay, starting with their second stay - up to a total of 50,000 bonus points, between August 15, 2011 and November 15, 2011, at participating Marriott properties.

Registered for both, and I don't see a reason I can't double dip. Didn't get an e-mail for either of them, so check Promotion Central. I'm guessing it's another targeted promotion. Seems to be the way Marriott is going lately.