Maybe it wasn't luck. The crystals could have been put at each end of the saber (and judging by the diagrams from those technical books, they were).
And yeah, it is just two lightsabers glued together. Each side is independant obviously, because if they shared any components at all, both sides would fail when Obi-Wan sliced it.
well..consider it luck, or will of the force, but the cut opnly disabled one half of his saber..only one crystal. The crystal as an energy cell, IF it's canon, seems a bit overdoing it in my opinion. you might as well have a power unit besides the crystal which i though only works as a lens for concentrating the beam. That way..it would work kind of like your mobile phone.
it would be easier to explain how a lot of jedi could die in ROTS. evil dooku, besides erasing kamino from the archive, storms the jedi temple and grtabs whichever saber chargers he can find.
"out of power, i am" moans yoda "my lightsaber charger, find, i cannot. how embarassing."