There are not two swords here, adventurers, despite the appearances. These two weapons are in truth one sword, separated but still whole, and with a deeper connection than you realise.
I shall tell you the tale...
...of two young, reckless and daring mercenaries who suffered deeper than any man should suffer.
The first went by the name Taefen Alemic, of Tharen's Hall. He was a skilled saber fencer and archer, and left home at a young age with his family's blessing to join the Mercenary Lord Illuthid's command.
The second was Parrus Sall-Duren of Garventon. He was a daring and oft-lucky troublemaker who stole a rapier and ran from his foster family at an early age and sought employ in Merc-Lord Illuthid's command as well.
This was a time of war. Merc-Lord Illuthid worked under the Regent Humphery Lacedon, who commanded the kingdom of Oryphed at the edge of the Unknown Lands. Regent Lacedon had sent his troops into battle with Perhiza, a Dark Lord of an ancient and evil god, now long forgotten. Troops clashed on battlefields across the lands, with nary a care who died so long as blood was spilled.
Parrus and Taefen met on in training, and rapidly became inseparable. Taefen was the perfect accomplice for Parrus' pranks, while Parrus was kept from committing anything too dangerous by Taefen's stability. They fought better together than apart, as their styles naturally complemented eachother's and did so yet better as they practiced together. Parrus learned archery from Taefen, while Taefen was aided by Parrus in the art of social interactions... most specifically, wooing women.
On the battlefield, they were a force that no sane man attempted to reckon with. Despite their reputation as childish troublemakers, they climbed the ranks faster than any before them, and soon were stationed as honorary captains sans command, and were often the leaders into large battles.
It was as a gift for these men that Merc-Lord Illuthid commanded the forging of two blades, one a rapier and the other a saber, of master worksmanship. In these days, master worksmanship meant forged in Silverflame, and such blades were unbreakable and inherently magical. On Taefen's request, backed by Parrus, the blades were forged simultaneously, in the same fire, and from one large bar of ore.
The swords were forged as a single blade until the final steps of creation, where they separated flawlessly and became the blades known as Cloud Dancer and Sky Singer.
I guarentee you, young adventurers, those blades were meant to fight together, and together they shall fight. And you shall find them powerful as no other, so long as they fight together.
The wielders of the Brothers must enter combat, each wielding his Brother, and fight together to a victory. Once they have, upgrade the blades to a +2 modifier so long as they are within 50' of eachother.