Did I fry my DUE by powering it from a external source and USB at the same time? If so, why?
If it was the same sketch involved in the destruction of both it may be a good idea to check to make sure the sketch that is involved as well to make sure there are no logical shorts to ground or other pins. Rugged-Circuits has a list of some common ways the Arduino can get shorted. It is showing the Uno but I am sure all similar rules apply with the exception of total power requirements and other differences between the boards.