* Shadow Pokémon don't faint: Instead of fainting, they become "purified" when they reach 0 HP. Purification removes their Shadow status and allows them to be used normally.
* Purification isn't a "knockout": While purification is similar to fainting, it's not the same. The Pokémon doesn't disappear from the battle or become unusable like a fainted Pokémon.
Therefore, you can't get a Shadow Pokémon "knocked out" twice in Pokémon XD.