Cut phyllo sheets into 13- x 9-inch rectangles; reserve half of phyllo sheets, keeping covered with a damp towel to prevent drying out. Stack remaining 8 sheets in a lightly greased 13- x 9-inch baking dish, lightly coating each sheet with vegetable cooking spray.
Bake on lowest oven rack at 400° for 5 minutes or until lightly browned, and set aside.
Peel shrimp, and devein, if desired.
Drain spinach well, pressing between paper towels.
Melt 1 tablespoon butter in a large skillet over medium heat; add garlic, and sauté 2 minutes. Add shrimp and scallops; cook 5 minutes or just until shrimp turn pink. Stir in cream cheese and next 4 ingredients until blended; remove from heat. Stir in spinach.
Melt remaining 1/4 cup butter in a large saucepan over medium heat. Add flour, whisking constantly; cook, whisking constantly, 1 minute. Gradually add 2 cups half-and-half; cook, whisking constantly, 3 minutes or until mixture is thickened. Stir flour mixture into shrimp mixture. Spoon into prepared baking dish.
Stack reserved phyllo sheets, coating each sheet with cooking spray. Roll up, jellyroll fashion, starting at long end, and cut into 1/4-inch slices. Unroll each piece, and gently twist; arrange twists in a diamond pattern over casserole.
Bake at 400° for 14 minutes or until golden. Let stand 10 minutes.