The cooking time is determined by the weight of one bird—not the combined weight. Use the weight of the smaller bird to determine cooking time. Use a food thermometer to check the internal temperature of the smaller bird first and then check the second bird. A whole turkey is safe when cooked to a minimum internal temperature of 165 °F as measured with a food thermometer. Check the internal temperature in the innermost part of the thigh and wing and the thickest part of the breast. When cooking two turkeys at the same time make sure there is enough oven space for proper heat circulation.
Source: FoodSafety.gov
