Melt 1/4 cup butter in a large saucepan over medium heat. Cook and stir red bell pepper, green bell pepper, onion, jalapeno pepper, Anaheim pepper, garlic, and oregano in the melted butter until peppers are tender, about 5 minutes. Season with salt, white pepper, and cayenne pepper.
Pour half-and-half into pepper mixture and bring to a boil, scraping any brown bits from the bottom of the pan. Reduce heat to medium-low; simmer until slightly thickened, about 3 minutes.
Slowly stir 1 cup Monterey Jack cheese into the cream sauce until melted, about 5 minutes; remove from heat.
Stir sour cream into the sauce.
Preheat oven to 350 degrees F (175 degrees C).
Melt 1/4 cup butter in a large skillet over medium-high heat. Cook and stir shrimp, scallops, and 1/2 cup green onion in melted butter until shrimp begin to turn pink, about 2 minutes.
Stir 1 chopped tomato and half the cream sauce into shrimp mixture.
Spoon about 1/3 cup shrimp mixture onto each tortilla. Roll tortilla tightly around the filling to form an enchilada. Arrange enchiladas seam sides down in 9x16-inch baking dish.
Pour remaining sauce and any remaining shrimp mixture and over enchiladas.
Cover baking pan with aluminum foil.
Bake in the preheated oven until bubbly, about 30 minutes. Remove foil and sprinkle enchiladas with 2 cups Monterey Jack cheese.
Return baking dish to oven; bake until cheese is melted, about 5 minutes.
Scatter 1/2 cup green onion and remaining chopped tomato over enchiladas to serve.
