Preheat oven to 350°. Combine first eight ingredients; beat on low speed 30 seconds. Increase speed to medium; beat 2 minutes. Pour into two greased and floured 8-in. round pans. Bake until a toothpick inserted in the center comes out clean, 28-32 minutes. Cool 10 minutes before removing from pans to wire racks to cool completely., Meanwhile, for grapefruit filling, combine sugar, cornstarch and salt in a large saucepan. Stir in water and grapefruit juice until smooth. Cook and stir over medium-high heat until thickened and bubbly. Reduce heat to medium; cook and stir 2 minutes longer. Remove from heat; gently stir in butter. Cool to room temperature without stirring. Cover and refrigerate filling until cold., Place one cooled cake layer on a serving plate; top with half of cooled grapefruit filling. Repeat layers., For frosting, beat cream cheese, sugar, juices and orange zest until blended. Gently fold in whipped cream and 3/4 cup coconut. Frost sides of cake; if desired, lightly frost top of cake, covering grapefruit filling. Refrigerate until cold, 3-4 hours. Sprinkle with toasted coconut if desired. Top with orange and grapefruit slices.