Preheat the oven to 400 degrees F. Cut the zucchini in half lengthwise and scrape out the flesh so they resemble boats. Place the zucchini boats in a 9-by-13-inch baking dish. Chop the flesh and set aside.
Heat the olive oil in a medium skillet over medium-high heat. Add the sweet and hot Italian sausage and cook, breaking up the meat, until no longer pink, about 4 minutes. Add the tomatoes, onion, garlic and chopped zucchini flesh. Season with salt and pepper. Cook, stirring, until the vegetables are softened, about 4 minutes.
Combine the parmesan, mozzarella, breadcrumbs and parsley in a medium bowl. Mound the sausage mixture into the zucchini boats. Sprinkle the breadcrumb mixture over the top. Bake until the zucchini is tender and the topping is golden, 20 to 25 minutes.