Add the uncooked cellentani pasta, cubed butter, cubed cream cheese, kosher salt, cayenne pepper, whole milk, and heavy cream to a 6-quart slow cooker.
Stir the mixture gently to distribute the ingredients.
Cover and cook on LOW for 1 hour. After 1 hour, stir the pasta and check for tenderness. If the pasta is not yet al dente, continue cooking for an additional 15 to 60 minutes, checking every 20 minutes to prevent overcooking.
Once the pasta is tender and most of the liquid is absorbed, stir in the freshly shredded cheddar and pepper jack cheese.
Cover and cook for another 10 to 15 minutes, or until the cheese is fully melted.
Stir vigorously to create a smooth, creamy sauce and serve immediately.