Add the uncooked cellentani pasta, sliced butter, cubed cream cheese, kosher salt, cayenne pepper, whole milk, and heavy cream to a 4-quart or larger slow cooker.
Stir the mixture gently to combine ingredients and ensure the pasta is mostly submerged.
Cover and cook on LOW for 1 to 2 hours. Stir once halfway through cooking to prevent sticking.
Check the pasta after 1 hour; it should be al dente (tender but with a slight bite).
Once the pasta is cooked, sprinkle the freshly shredded cheddar and pepper jack cheese over the top.
Cover and cook for an additional 10 to 15 minutes until the cheese is fully melted.
Stir vigorously to blend the melted cheeses into the sauce until smooth and creamy. Serve immediately.