Preheat oven to 350 degrees F. Grease a casserole dish and set aside.
Peel the sweet potatoes and cut them into 2 - 3 inch pieces. Place the prepared sweet potatoes in a covered, microwave-safe dish. Microwave the sweet potatoes for 15 - 18 minutes, or until the sweet potatoes are tender. Pause the microwave and stir the potatoes halfway through cooking time. Mash the sweet potatoes with a potato masher.
In a clean bowl, combine the mashed sweet potatoes, pumpkin, 1/4 cup of brown sugar, 4 tablespoons of melted butter, beaten egg, vanilla, pumpkin pie spice and salt. Mix very well to combine the mixture completely.
In a separate bowl, combine the remaining 1/3 cup brown sugar, flour, pecans and pinch of salt. Stir together with a fork. Pour in the remaining 2 tablespoons of melted butter. Mix well to combine.
Spread the sweet potato and pumpkin mixture evenly into the prepared casserole dish. Sprinkle the pecan topping evenly over the filling.
Bake, uncovered, for 25 - 30 minutes.