I`m Fully agree with your statement rcooper but now in a practical case it would be as having a flow from a less diameter pipe to a larger diameter pipe. And to be more specific from a D diameter pipe to a 2D diameter pipe.
We can solve that and see what happend with pressure, velocity and...
As i see them both are necessary, as you deal with an M classified is important to guaranty both soundness and stress resistance that may appear to be the same but not always are the same.