Pour the uncooked cellentani pasta into the bottom of a 6-quart slow cooker.
Scatter the butter slices and cream cheese cubes evenly over the pasta.
Sprinkle the kosher salt and cayenne pepper over the top.
Pour the whole milk and heavy cream over the mixture. Stir gently to combine and ensure the pasta is in an even layer coated by the liquid.
Cover and cook on LOW for 1 to 2 hours. Start checking at the 1-hour mark; the pasta should be cooked to al dente (tender with a slight bite) and most liquid absorbed.
Once the pasta is cooked, add the freshly shredded cheddar and pepper jack cheeses.
Stir constantly until the cheese is fully melted and the sauce is smooth and creamy. Serve immediately.