The Darkness Before the Dawn (Dark Sun: Chronicles of Athas, #2)

Jedra lives in the streets, Kayan in the temple. When misfortune throws them together, they discover psionic power greater than almost any other on Athas, a power potent enough to destroy whole cities. When the force proves too strong for them to control, they search for a mentor to help them harness their wild talent and use it for the good of their ravaged world.