Thanks for the reply...I suspected the answer was not simple.
It was never my intent to parallel the regulators. In my opinion, the shield was poorly designed (and even more poorly documented) to allow this to happen accidentally. I just didn't understand why the Arduino and L298N, both faced with the same 'dual source', reacted differently ("ho hum" vs. "KABOOM" respectively). Suffice it to say that when I receive my replacement shield I will not make the same mistake, regardless of whether I understand or not why it is a mistake!