Sauté onion and garlic in oil in saucepan. Stir in enchilada sauce and tomato sauce. Season with salt and pepper.
Place two tortillas in bottom of slow cooker. Layer one-third chicken on top. Top with one-third sauce mixture, one-third beans, one-third corn, one-third cheese, and one-third black olives. Repeat layers 2 more times. Top with 2 tortillas.