Heat oven to 350 degrees F. Coat a 3-1/2-quart oval dish with nonstick cooking spray.
Heat oil in a large nonstick skillet over medium-high heat. Add scallops, shrimp, 1/4 teaspoon of the salt and the pepper. Cook for 2 minutes per side. Remove to a plate.
Blend half-and-half and cornstarch. Bring to a simmer in the skillet. Simmer 1 minute. Take off heat and stir in clams and the juices, 1/2 cup of the cheese, the tarragon, cayenne and remaining 1/4 teaspoon salt. Add cooked shellfish and toss with fettuccine.
Spoon into prepared dish. Sprinkle with remaining 6 tablespoons cheese. Bake at 350 degrees F for 20 minutes.