Preheat oven to 500°, with racks in the middle and lower thirds. On a lightly floured surface, roll dough into two 14-inch ovals and transfer to two parchment-lined rimmed baking sheets. In a large bowl, toss potatoes and shallots with 2 teaspoons oil and season with salt and pepper. Arrange potatoes and shallots on dough, leaving 3/4-inch border. Brush borders with 2 teaspoons oil and bake 5 minutes.