I have asked this question of someone at Marriott (don't remember who, probably the reward desk email) and you are correct, it has to be 2 3-night stays.

However, it never hurts to call the property and ask the sales manager. Depending upon how the properties get reimbursed from Marriott, he may be willing to make an exception and then run it through to Marriott however he has to in order for it to "look legit".