Soak wheat overnight, to soften. Wash rice by placing it in a medium saucepan, covering with water, and swishing it gently with your hand to release some of the starch. Drain rice, then repeat the process twice.
Add pearl barley, the softened wheat, a large pinch of salt, and 4 1⁄2 cups water to the saucepan. Bring to a boil over medium heat. Reduce heat to the lowest setting. Simmer partially covered, keeping the lid slightly ajar with a chopstick, for about 1 1⁄2 hours, stirring often. The consistency of the finished congee should be that of a thin porridge. Add more water during cooking if it becomes too thick.