Boil the cavatappi in water with 1 teaspoon salt for 3 minutes less than the package directs and drain.
Butter a 6-quart crockpot and add the pasta, evaporated milk, whole milk, Cajun seasoning and remaining salt.
Cook on low for 45 minutes, then stir in the cheddar and pepper Jack and cook for 40 minutes more.
Heat the oil in a skillet and cook the shrimp for 2 to 3 minutes per side until opaque and 145°F, then add the lemon juice.
Fold the shrimp into the macaroni and cook on high for 10 minutes until the whole dish is hot.