Heat oven to 400°F. Make pie crusts as directed on box for Two-Crust Pie, using 9-inch glass pie plate.
In large bowl, mix gelatin, cornstarch and 1 cup sugar; stir in peaches. Spoon into crust-lined pie plate. Top with plum slices. Dot with butter. Top with second crust; seal edge and flute. Cut slits in several places in top crust.
Brush small amount of water over top crust; sprinkle lightly with 1/2 teaspoon sugar. Cover edge of crust with strips of foil to prevent excessive browning.
Bake 10 minutes. Reduce oven to 350°F; bake 35 to 40 minutes longer or until crust is golden brown and peaches are tender. Cool on cooling rack 1 hour 30 minutes before serving.