Preheat your oven to 350°F (175°C).
Cook the spaghetti according to the package instructions. Drain and set aside.
In a large skillet, melt the butter over medium heat. Add the diced onion and bell pepper, and cook until softened, about 5 minutes. Add the minced garlic and cook for another 1 minute.
Stir in the shredded chicken, cream of chicken soup, cream of mushroom soup, Ro-Tel tomatoes, chicken broth, and sour cream. Mix until well combined.
Add the chili powder, cayenne pepper (if using), salt, and pepper. Stir to combine.
Gradually add the cooked spaghetti to the mixture, stirring to coat the pasta evenly with the sauce.
Pour half of the spaghetti mixture into a greased 9x13 inch baking dish. Sprinkle with half of the cheddar and mozzarella cheese.
Add the remaining spaghetti mixture on top and sprinkle with the remaining cheese.
Bake in the preheated oven for 25-30 minutes, or until the cheese is melted and bubbly.
Remove from the oven and let it sit for a few minutes before serving.