Preheat oven to 350 degrees and spray a 9×13-inch baking dish with cooking spray.
Cook pasta to al dente according to package directions. Be sure to salt the water. Drain, return to pot, and set aside.
Melt 2 tablespoons of butter in a large skillet. Add shrimp. Sprinkle with Cajun seasoning and cook for about 2 minutes per side. Note: Shrimp will cook more in the oven so undecook them a little. Remove shrimp from skillet and set aside.
Add 3 tablespoons of butter to skillet and place over medium heat. When melted, add flour. Cook and stir for 1 minute.
Gradually whisk in whole milk and then evaporated milk. Bring to a simmer and simmer for 2 to 3 minutes. Add Cajun seasoning, garlic powder, and black pepper.
Turn Heat to low. Set 1/2 cup of cheddar cheese and 1/2 cup of Colby Jack cheese aside. Add the remaining cheese a handful at a time to the skillet, stirring until melted before adding the next handful.
Stir in crab and shrimp. Pour cheese mixture over pasta. Stir to mix. Transfer to prepared baking dish.
Sprinkle the 1 cup of cheese that was set aside on top. Combine topping ingredients and sprinkle on top.
Bake for 20 minutes. If you want it a little more golden on top, broil for a few minutes.
