Fill a heavy bottom skillet with ½ inch oil and preheat to 350°F.
Place garbanzo beans, onion, garlic, cilantro, and parsley in a food processor and process until finely ground.
Scoop mixture into a mixing bowl and add remaining falafel ingredients and stir together until fully combined. Cover with plastic wrap and place in refrigerator for about 2 hours.
For tzatziki: Place oil, vinegar, cumin, and garlic in a small mixing bowl and whisk together. Add yogurt and cucumber to the mixture, season with salt and pepper and continue to whisk together. Set aside.
Form 2 tablespoon sized patties with the falafel mixture and fry for 3 to 4 minutes. Flip each patty and fry for an additional 2 minutes. Drain on paper towels and season with salt and pepper.
To assemble: Smear the insides of each pita with a small amount of tzatziki and fill with 3 falafels, a small amount of lettuce and a sprinkle of diced tomatoes. Top with more tzatziki and serve.