In a medium saucepan of boiling water, cook the green lentils over moderately high heat until almost tender, about 15 minutes. Add the red lentils and continue cooking until all the lentils are tender, about 8 minutes longer. Drain the lentils. Return them to the saucepan and let cool to room temperature. Stir in the scallions, sun-dried tomatoes, olives, lemon juice and olive oil. Season with salt and pepper and serve.
Make Ahead: The salad can stand at room temperature for up to 4 hours.