In a 3-quart saucepan, mix lemon juice and cornstarch until smooth. Stir in blueberries and 2/3 cup sugar; heat to boiling over medium-high heat. Reduce heat to medium; cook 1 minute. Stir in peaches then set aside.

Preheat oven to 425°F. In a bowl mix flour, baking powder, salt, and 1/3 cup sugar. With pastry blender or two forks, cut in 9 tablespoons margarine until mixture resembles coarse crumbs.

Stir in milk, a little at a time, just until mixture forms a soft dough that leaves side of bowl. On a lightly floured surface, knead dough 6 to 8 times, just until smooth. With lightly floured hands, pat dough 3/4 inch thick.

With floured 3-inch round biscuit cutter, cut out shortcakes. With pancake turner, place shortcakes 1 inch apart on ungreased large cookie sheet. Press trimmings together; cut to make 8 shortcakes in all.

Melt remaining 1 tbsp margarine and brush over shortcakes. Sprinkle with 1 tbsp sugar. Bake 16 to 22 minutes, until golden brown. In a small bowl, with mixer at medium speed, beat cream with remaining 2 tablespoons sugar to soft peaks. With fork, split warm shortcakes in half. Spoon some fruit into each; top with cream, then more fruit.