Am I the only one who thinks that would actually be very interesting, not just as a vague concept, but as an actual RULE in the setting. If the Forces of good create a Holy Sword, they must also create an Unholy Sword with the exact same properties, same goes for the forces of Evil creating an Unholy Sword. If you destroy an item like this, it's "Twin" is also destroyed. Therefore, the forces of Evil keep building elaborate vaults in which to hide "Good" magic items (So they can use the Evil ones on the front lines), while the forces of Good do the same thing with the Evil items that they keep building while attempting to equip their own forces. If you manage to raid these vaults you get a bucketload of magic items that you can wield yourself, or destroy to deprive the enemy of the use of their twins.
Neutral, unaligned items can be created, but they can be used by either side, and can therefore be easily looted in battle.
Of course, this requires that the setting be based around some big cosmic struggle between "Good" and "Evil".
Unless each side refers to itself as the "Good" side, wielding "Holy" Weapons against their "Evil" foes, with each side excusing their crimes as "The Neccessities of War", while pointing to "The debased and vile nature of our foe" every time their enemy commits some atrocity.
Kinda reminds me of the dark crystal, where the skeksies and the mystics are two sides of the same coin, and when one is destroyed the other is.