I've thought and thought about this one. I'd think you can pretty much rule out a material problem because of the dual failure. The only thing that makes sense to me is a poor wheel build. If a number of spokes 180 deg opposite from each other were way too tight, and the ones 90 deg out from these were then too loose it would cause rim forces much like if you were to push down on a hula hoop. Every revolution of the wheel would be the equivalent of pushing and letting up on the hoop. The aluminum would fatigue pretty quickly at the two sides where the spokes were loose. When the first side failed catastrophicaly that would send a shock wave around the rim causing the failure at the opposite side.