This unique sword was crafted by a pair of brothers, each a master bladesmith. While the sword is split in two and a creature is attuned to a half, that half has the affect of a +1 Rapier (+1 bonus to damage and attack rolls), and you also know the direction of the sword's other half (but not the exact location). If two siblings are both attuned to each half and are weilding each half independently while attacking the same target within 5 feet, either sibling can use a bonus action to gain advantage on attack rolls against the target, and the target's attacks against either sibling within 5 feet are made with disadvantage until the end of the sibling who used their bonus action's next turn. No creature may benefit from both halves at the same time (instead each acts as a normal rapier). When the two halves are joined, the weapon has the effect of a Defender Longsword. 

Defender Longsword

You gain a +3 bonus to attack and damage rolls made with this magic weapon.

The first time you attack with the sword on each of your turns, you can transfer some or all of the sword’s bonus to your Armor Class, instead of using the bonus on any attacks that turn. For example, you could reduce the bonus to your attack and damage rolls to +1 and gain a +2 bonus to AC. The adjusted bonuses remain in effect until the start of your next turn, although you must hold the sword to gain a bonus to AC from it.

Proficiency with a Rapier allows you to add your proficiency bonus to the attack roll for any attack you make with it.

Notes: Bonus: Magic, Weapon (Rapier), Very Rare (requires attunement), Damage, Combat, Finesse, Vex