Preheat oven to 375 degrees, with racks in middle and lower thirds. In a large bowl, whisk together cornstarch and 1/2 cup granulated sugar. Stir in apricots and blueberries. Spread mixture into a 10-inch cast-iron skillet.
In a food processor, pulse remaining 2 tablespoons granulated sugar, flour, baking powder, and salt until combined. Add butter and pulse until mixture resembles coarse meal, with a few pea-size pieces of butter remaining. Add heavy cream and pulse 2 or 3 times until combined.
Spoon batter in 8 mounds on fruit mixture. Press lightly on tops to flatten, brush with heavy cream, and sprinkle with sanding sugar if desired.
Bake on middle rack, with a foil-lined rimmed baking sheet on lower rack to catch drips, until biscuits are golden and juices bubble in center, 40 to 45 minutes. Transfer to a wire rack and let cool. Serve warm or at room temperature.
Cover and keep at room temperature, up to 2 days.