Preheat oven to 400°F and wash potatoes; pierce each several times with a fork.
Place potatoes directly on oven rack and bake for 45 to 60 minutes until tender; let cool slightly.
In a medium bowl, mix chopped shrimp, crab meat, Cajun seasoning, garlic powder, paprika, salt, and pepper.
Stir in sour cream and one-third of shredded cheddar cheese until combined.
Slice potatoes lengthwise and scoop out most of the flesh, leaving a 1/4-inch shell.
Mash potato flesh with butter and milk or cream until smooth.
Gently fold seafood mixture and half of the remaining cheese into mashed potatoes.
Fill potato shells with the seafood potato mixture and sprinkle remaining cheese on top.
Reduce oven to 375°F, arrange stuffed potatoes on baking sheet, and bake for 12 to 15 minutes until cheese melts and tops are golden.
Garnish with green onions or parsley if desired and serve hot immediately.