I had two 2-night stays this weekend (Th-Sat and Sat-Mon), using the Bonus Buck promo requiring a Fri or Sat night stay. I used the BB from the first stay on the second and talked to the desk on Saturday morning to keep me in the same room. I've done it several times without problem.

The only time I could see a problem is if you were upgraded to some super room on your first stay, without telling them you had continguous stays, and they were sold out of that room type with paid stays over the second set of nights.