Just so it is not lost. You only get elite night credit for one room. Be careful if you overlapping stays that they have same start or end. Otherwise you might not get full credit.
For instance if you start on day 1 in one room A and the start on day 2 in room B. Then check out of room A on day 3 and room B on day 14, Marriott system will only get 3 nights credit since it uses first stay as the "real" stay. No issues if Room A and Room B have same start and finish. But you only get credit for one night per day since you can only be in one place at a time.
Also, even if they overlap, I would suggest splitting credit so someone else is getting the nights on second room Elite night credit.