Bring the half-and-half to a simmer in a large saucepan over medium heat. Whisk in the American cheese and cream cheese until smooth. Remove from the heat and whisk in the Cheddar, mustard, paprika, 1 teaspoon salt and several grinds of black pepper. Put 1/2 cup sauce each into 2 medium bowls. Pour the remaining sauce into a 13-by-9-inch baking dish, spreading it into an even layer.
Cook the large shells according to the package directions. Transfer with a slotted spoon to one of the medium bowls and toss to coat with the sauce. Cook the small shells in the same pot of water according to the package directions. Drain well, then transfer to the remaining medium bowl and toss to coat with the sauce.
Preheat the oven to 425 degrees F. Spoon the small shells into the big shells and line them up neatly in the baking dish, nestling them in the cheese sauce. Sprinkle with the cheese crackers. Bake until hot through, bubbling and the crackers are starting to brown, about 15 minutes. Let cool 5 minutes before serving.
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