In a medium saucepan, cover wheat berries with a few inches of water; bring to a boil. Remove from heat and let stand for an hour. (Alternatively, soak them in water overnight.)
Pour off most of the water from the wheat berries, add the barley to the pot and cover with water by a few inches; bring to a boil and cook for 40 minutes, until both barley and wheat berries are tender. Drain and rinse under cold water to stop them from cooking; drain well and transfer to a bowl. Stir in the dates and let cool completely.
Add the chickpeas, feta, celery, parsley, onion and walnuts; drizzle with oil and vinegar and sprinkle with salt and pepper; toss to combine. Serve immediately or refrigerate until needed.