Mix onion, 1 tablespoon lemon juice, oil, garlic, cumin, coriander, paprika, cinnamon, salt and pepper in a 4-quart slow cooker. Add chicken and coat thoroughly.
Cover and cook on low for about 6 hours until the chicken reaches at least 165°F and is tender.
Lift chicken onto a foil-lined metal sheet pan, leaving excess juices behind. Pull into broad strips and broil 6 inches from the heat for 3 to 5 minutes until a few edges brown, watching constantly.
Stir yogurt with the remaining tablespoon lemon juice. Keep cucumber and tomato cold until serving.
Warm pitas in a dry skillet over medium heat for about 30 seconds per side and open each pocket.
Fill each pita with one-quarter of the chicken, cucumber, tomato and yogurt sauce.