Boil, bake, or nuke the potatoes. Cut potatoes in half and arrange in a large casserole dish - leave skins on potatoes - press down with a fork. Spread butter over potatoes. Spread Bacon bits over butter. Add Salt and pepper. Devein and peel shrimp and crawfish. Sauté shrimp and crawfish in butter with garlic and cajun seasoning - add to casserole. Spread grated cheese over top. Sprinkle cajun seasoning on top. Bake until hot and cheese is melted. Top with chopped green onion tops.