Wind by Naomi kritzer

Once upon a time, there were two young girls, closer than sisters, who dreamed of greatness. When they played together (as they did every day), Gytha always pretended to be an artist, raising glorious sculptures of stone and glass, and Dagmar pretended to be a famous physician, making brilliant discoveries each day and then spending her nights in the slums, secretly healing those too poor to afford a physician’s fee.
Magical ability comes from an imbalance of the elements within the human heart, however, and both Gytha and Dagmar had been blessed with balance rather than power. But Gytha read in a book about a perilous rite requiring two willing hearts that would allow them to trade elements, creating an imbalance and leaving both people with magical power. “Give me your Air,” Gytha suggested to Dagmar, “and I will give you my Earth.”