Cut phyllo sheets into 13- x 9-inch rectangles; reserve half, 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 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 ¼ 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 with cooking spray.
Roll up, jelly-roll fashion, starting at long end, and cut into ¼-inch slices.
Unroll each piece, and twist; arrange twists in a diamond pattern over casserole.
Bake casserole at 400° for 14 minutes or until golden.
Let stand 10 minutes.
