Stir yogurt and 1 cup salsa in medium skillet. Add chicken and 1/2 cup corn, cook over medium-high heat until chicken is cooked. Simmer sauce 10 more minutes, until reduced to 1 1/2 cups.
Meanwhile, remove one breast to bowl. Use two forks to shred; repeat with remaining breasts. Keep warm until sauce is reduced. Toss sauce and chicken together. Season with salt and pepper.
Place beans in small saucepan. Add 1/4 cup water and cook over medium heat until warm. Stir, pressing beans against pan to smash. Season with lime juice, salt and pepper.
In small bowl, combine remaining salsa and corn. To assemble, spread tostadas with beans, top with chicken, lettuce and cheese. Garnish with additional yogurt and salsa.