Hi mig,
I'm certainly no expert as I just recently decided on BU, but it seems that if you take a few of your core classes in the summer before you matriculate, you can get away with taking your masters exam in the spring.
You'd be done with classes at this point, and you could be finished with your MPH depending on when you do your practicum (end of spring semester if you do your practicum then, or late June/early July if you do it early in the summer).
I would suggest talking to the Epi cirruculum coordinator (she's extremely helpful and very easy going), but this was basically what she told me.
Hope that helps!