Tom,
My suggestion is to enable the full DS3 and have each router traffic-shape to 6M on the neighboring interfaces. Rate-limit on the input of your router to disallow the customer from sending you more than the limit. Remember that for the most part rate-limiting polices, where traffic shaping performs more buffering and shaping. That’s why you should use the combo of a shape on the sender side and a police on your side to protect.
–Dan