Heat butter and oil in a 10-inch heavy skillet over moderate heat until foam subsides, then cook garlic and bread crumbs, stirring, until crumbs are golden. Transfer to paper towels to drain and season with salt.
Preheat oven to 350°F. Chop chipotles. Melt butter in a 3- to 4-quart saucepan over moderate heat, then add flour and cook, whisking, 1 minute. Gradually whisk in milk, cream, and mustard and simmer, whisking occasionally, 3 minutes.
Cook macaroni in a 6- to 7-quart pot of boiling salted water until just tender. Drain in a colander and transfer to a large bowl. Stir in white sauce, cheese, and salt to taste.
Fill a 1 1/2-quart shallow casserole with half of macaroni mixture. Stir chipotles into remaining macaroni. Spoon into another 1 1/2-quart shallow casserole and sprinkle both with bread crumbs.
Bake casseroles in middle of oven 30 minutes, or until bubbly.