Lay tortillas on a flat surface. Place cheese, crab, and shrimp in the middle of each (ration the ingredients so each tortilla will contain an equal amount) Make sure to set aside some cheese to sprinkle on top of the tortillas. Roll the tortillas so that the seafood, cheese mixture is held securely inside.
Place rolled tortillas side by side in a 9x13 inch baking pan.
Pour green enchilada sauce over all the enchiladas, covering them completely. Sprinkle the remaining cheese over the enchiladas.
Cover and bake at 350 degrees for 30 minutes.
Uncover the enchiladas and cook an additional 15 minutes.
Serve hot, garnished with sour cream and green onions.