Blades of Light and Dark (3.5e Equipment)
From D&D Wiki
Blades of light and dark are twin blades, both were created at the same time by two master blacksmith brother's trying to prove they were the better blacksmith. one brother was part cleric, and the other a wizard. when the two brother's finally finished their swords, they noticed that both swords were identical in shape and size. and when the two weapons were placed side by side to be examined they joined at the hilt like Siamese twins. they also noticed that it seemed as though the two swords could be combined to become what looked like a double bladed sword. however when they attempted to connect them a string of magical energy appeared in between the tips of the two swords making it into a bow.