Heat gas or charcoal grill. To flatten each chicken breast, place between 2 sheets of plastic wrap or waxed paper. Working from center, gently pound chicken with flat side of meat mallet or rolling pin until about 1/4 inch thick; remove wrap.
When grill is heated, carefully oil grill rack. Place chicken on gas grill over medium heat or on charcoal grill over medium coals; cover grill. Cook 6 to 8 minutes, turning once, until chicken is no longer pink in center. During last 5 minutes of cooking time, heat bacon on grill, turning once, until hot. If desired, during last 3 minutes of cooking time, toast kaiser roll pieces, cut side down, on grill.
Spread cut sides of roll pieces with mayonnaise. Cut each chicken breast into 2 pieces. For each sandwich, on bottom section of roll, place 2 chicken pieces, overlapping if necessary. Top with lettuce leaf, tomato slice and middle section of roll. Place 2 Canadian bacon slices on top of middle roll section. Top with lettuce leaf and top section of roll. Press each sandwich slightly; spear each with 2 long toothpicks. Cut sandwiches in half between toothpicks.