1. Preheat oven to 375°F (190°C) and grease a 9×13-inch baking dish.
2. Cook pasta according to package directions until al dente. Drain and set aside.
3. In a large bowl, mix shredded chicken with buffalo sauce and ranch dressing until well coated.
4. Combine the cooked pasta with the buffalo chicken mixture and stir in half the mozzarella and cheddar cheese.
5. Transfer the mixture to the prepared baking dish. Sprinkle the remaining mozzarella and cheddar cheese over the top.
6. Bake for 20-25 minutes, or until the cheese is melted and bubbly.
7. Garnish with chopped green onions and serve hot.