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.