In a medium saucepan, combine marinara sauce, Alfredo sauce, and 1/2 teaspoon Italian seasoning. Simmer for 10 minutes, stirring occasionally.
Cook the tortellini according to package directions, using the shorter end of the range. Drain well.
Stir the drained tortellini into the sauce until coated. Transfer to a greased 9-inch pie dish or similar baking dish.
Top with mozzarella, Parmesan, and the remaining Italian seasoning.
Broil 1 to 3 minutes, watching closely, until the cheese bubbles and develops golden spots.
Let rest 2 to 3 minutes before serving.
Notes
Watch closely under the broiler because the cheese can go from golden to burned quickly. Do not rinse the tortellini after draining because the starch helps the sauce cling. Refrigerated tortellini cooks faster, but dried tortellini also works. Let the casserole rest briefly before serving so it scoops more cleanly.