Combine yogurt, garlic, lemon juice, cayenne and salt in a small bowl. Cover and refrigerate until using.
Combine chickpeas, parsley, cilantro, garlic, scallions, flour, cumin, coriander, salt and pepper in a food processor. Run until well combined. Transfer mixture to a bowl. Form into 12 falafel balls.
Heat oven to 200 degrees F. Heat 1 tablespoon of the oil in a large nonstick sauté pan over medium to medium-high heat. Add 6 falafel to pan and brown 2 minutes per side, about 8 minutes total. Transfer to 200 degrees F oven to keep warm. Repeat with second batch.
Serve falafel in a pita half with tomato, cucumber and Spicy Yogurt Sauce.