In a large mixing bowl, combine flour, 1/4 cup sugar and salt. Cut in butter and shortening until mixture resembles coarse sand. Add buttermilk and mix just until dough comes together in a ball. Divide dough evenly into thirds; wrap in plastic and refrigerate 2 to 3 hours.
Heat oven to 325°. Roll each piece of dough on lightly floured parchment to a 10-inch circle. Prick with a fork and sprinkle liberally with sugar. Bake at 325° for 30 minutes.
To assemble: Set aside a few strawberries and blueberries to top finished dessert. Place one pastry disk on a serving plate and cover with a third of the remaining berries, followed by a third of the whipped cream. Repeat layering two more times and finish with reserved berries.