In my campaign, the players have recently uncovered information about an unknown branch of the Order of Santiago which was not secularized in the sixteenth century, but instead continues to watch over four Swords of the Saints: Joyeuse, Durandal, Curtana and Almace, and their potential wielders. [Note: I chose these with reference to the Song of Roland and each contains relics of a specific patron saint.]
Of course, Charlemagne’s sword is said to have been buried with him, and Curtana is currently in Ultima Thule… 
We did something similar in my game. I even have stats for Durendal, if you're interested.
Personally, I don't think it breaks anything if there are more swords, though said swords should have different powers. Here's what I have for Durendal.
Durendal [-3]Description: Durendal is a holy relic every bit as powerful as one of the Swords of the Cross. While the Knights of the Cross concern themselves with fighting the Order of the Blackened Denarius, the weilder of Durendal (and several other lesser holy relics) has been tasked with protecting a smaller area of the world. Where the Swords of the Cross can cut through the most potent protections Durendal can temporarily strip an evil entity of its greatest power. Such a boon does not last long, however, and so the weilders of this holy relic learn to finish their fights quickly.
Musts: You must have a destiny or calling to inherit the Sword, represented as a high concept or template.
Skills Affected: Weapons, others
Armor of Faith. When weilded with a true and holy purpose Durendal surrounds the bearer with a nimbus of golden radiance. This holy light provides the weidler with Armor: 2 against the attacks of supernaturally evil beings and creatures.
Divine Purpose. Durendal may only be swung with true selfless purpose in mind and heart; if this is not the case, the bond between the Weilder and the Sword is broken and may only be restored by undergoing some sort of trial of faith. When swung without such purpose in mind and heart, the blow does not land (any attack roll automatically fails), the bond is immediately broken, and the sword falls from the wielder’s hand.
Hidden from the Unworthy. Durendal was only meant to be weilded by those of great faith and piety. Those who do not possess the Righteousness power tend to overlook the presence of Durendal – at least until it has been drawn.
Holy. This weapon is a powerful holy symbol in its own right. Its very touch is like holy water or that of a cross or other symbol of faith backed by the belief of the possessor.
Lay Low the Wicked. Durendal was crafted with a singular purpose in mind – protecting the innocent from supernatural evil. The Weilder of Durendal can temporarily supress the powers of supernaturally evil beings by spending a Fate point and making a special maneuver. This is a Weaponry manuever opposed by the target's Conviction or Endurance (whichever is higher). If the maneuver is successful the Weilder may supress one of the target's powers for a number of exchanges equal to the shifts gained in the action. If the maneuver is unsuccessful the Weilder instead takes stress equal to the shifts gained by the target.
It’s a Sword. Durendal is a scimitar (+2 damage) with a golden hilt. It is said to contain within its golden hilt one tooth of Saint Peter, blood of Saint Basil, hair of Saint Denis, and a piece of the raiment of the Blessed Virgin Mary.
Unbreakable. As an Item of Power, it cannot be broken, save through dedicated magical ritual predicated upon perverting its purpose.
Discount Already Applied. As an Item of Power, the sword already includes the one-time discount (page 167). This means that if the character possesses more than one Item of Power, the one-time discount will not apply on that second item. If Durendal is the second or subsequent artifact the character gains, the refresh cost is –5.
