DO-AHEAD TIP: Marinate chicken for 20 minutes (see recipe). If using bamboo skewers, pre-soak them in water for 30 minutes, to prevent burning.
In a large bowl, toss together chicken cubes, 4 tablespoons of olive oil, juice of 1 lemon, the garlic, ground cumin, paprika, crushed red pepper flakes, salt and pepper.
Cover and refrigerate for 20 minutes.
Preheat an outdoor grill or indoor grill pan to medium heat.
Thread chicken cubes onto skewers; grill for 8 minutes per side or until browned on the outside and juices run clear.
Meanwhile, prepare yogurt sauce: In a food processor or blender, place remaining 2 tablespoons of olive oil, the juice of the remaining lemon, the yogurt, salt and pepper; blend for 1 minute.
Serve sauce with chicken.