Heat closed contact grill 5 minutes. When grill is heated, place bacon on bottom grill surface. Close grill; cook 4 to 6 minutes, turning once, until crisp. Drain on paper towels.
Meanwhile, place 1 chicken breast, boned side up, between 2 sheets of plastic wrap or waxed paper. With flat side of meat mallet or rolling pin and working from center, gently pound chicken until 1/4 inch thick; remove wrap. Repeat with remaining chicken breast. Sprinkle chicken with Italian seasoning.
Place chicken on bottom grill surface. Close grill; cook 4 to 6 minutes or until fork-tender and juices run clear.
Spread toasted bread slices with mayonnaise. Layer lettuce, tomato, chicken and bacon between toast slices.