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