Combine flour, 2 tablespoons sugar, and salt; cut in 1/2 cup butter and shortening with pastry blender until mixture resembles coarse meal. Sprinkle ice water evenly over surface, stirring until ingredients can form a ball.
Turn dough out onto a lightly floured cloth. Roll into a 16- x 12-inch rectangle. Brush 1 tablespoon melted butter over surface of pastry.
Combine blackberries, raspberries, 1 1/2 cups sugar, and nutmeg, tossing lightly to coat well. Spread evenly over pastry.
Starting at long edge, roll up dough jellyroll fashion; press seam to seal. Cut berry roll in half; place each half on a lightly greased 10- x 6- x 2-inch baking dish. Bake at 400° for 20 minutes; reduce heat to 350°, and bake 50 to 55 minutes. Cool slightly, and cut into slices. Serve with a dollop of whipped cream.
Note: 2 (10-ounce) packages frozen raspberries, thawed and drained, may be substituted for fresh. One roll may be frozen.