David, I am not a fan of mechanicals either simply because if it moves, eventually it will fail. However, my dad shot a really nice 9-point 10-12 years ago with a 125gr thunderhead fixed blade, 3-blade broadhead, double lung exactly as you are describing it. That buck went over a half of a mile before we found it. We checked the wounds when gutting it and he had indeed gone clean through both lungs. I guess sometimes you just miss the major veins and arteries and they have to suffocate to death. Just because something is unlikely doesn't mean it never happens.