Spoon the marinara sauce over the pizzas, dividing equally and leaving a 1-inch border around each pizza. Sprinkle the mozzarella and Fontina cheeses over the pizzas, dividing equally. Sprinkle the mushrooms and pancetta over the cheese. Bake the pizzas until the crusts are crisp and brown on the bottom and the cheese is melted on top, about 15 minutes. Cut the pizzas crosswise into rectangular slices and serve immediately.