Grease the insert of a 6-quart slow cooker lightly with non-stick spray or butter.
Add the uncooked cellentani pasta, butter slices, cubed cream cheese, kosher salt, and cayenne pepper into the slow cooker.
Pour the whole milk and heavy cream over the ingredients. Stir well to combine and ensure the pasta is evenly distributed.
Cover and cook on LOW for 1 to 2 hours. Stir the mixture after the first hour to prevent sticking. Check for doneness around the 1.5-hour mark; the pasta should be tender (al dente) but not mushy.
Once the pasta is cooked, stir in the shredded sharp cheddar and pepper jack cheese.
Cover and cook for an additional 10 to 15 minutes, or until the cheese is fully melted and the sauce is creamy.
Stir gently before serving hot.