Over high heat, bring 2 gallons water, 1/4 cup seafood seasoning and 1/8 Cajun spice to a boil. Add the potatoes and corn and cook until potatoes are soft to the touch, 15 to 18 minutes. Next, add the onions and continue to cook for 2 minutes. Next, add the sausage and shrimp and cook until the shrimp are bright pink, a final 3 to 5 minutes. Remove from the water with a strainer.
Next, in a large bowl, blend the remaining 1/4 cup seafood seasoning, 1/8 cup Cajun spice with the butter. Whisk together and toss with the cooked boil, tossing to well coat. Serve.