Place the lime sherbet in the chilled mixer bowl. Using the paddle attachment, beat on low speed until ice cream is spreadable, about 30 seconds. Remove chilled mold from freezer. Using a rubber spatula, coat inside of mold with all the lime sherbet, making a 1/2-inch-thick layer. Return mold and empty mixer bowl to the freezer for 30 minutes.
Place the vanilla ice cream in chilled mixer bowl. Beat on low speed until spreadable, about 30 seconds. Remove mold from freezer. Using the rubber spatula, coat lime layer with the vanilla ice cream, making a 1/4-inch-thick layer. Return mold and empty mixer bowl to freezer for 30 minutes. Place sugar and 3/4 cup water in a small saucepan. Bring to a boil over medium heat, stirring occasionally. When sugar has dissolved, remove pan from heat,
Place sorbet in chilled mixer bowl. Beat on low speed until sorbet has softened, about 1 minute. Fold in wafer pieces. Remove mold from freezer. Pack sorbet into center, making an even layer 1/3-inch below ice cream. Cover sorbet portion with sponge cake; trim cake if necessary. Return mold to freezer until completely hardened.