In a large bowl, blend sweet potatoes, 1/3 cup butter, white sugar and 2 tablespoons brown sugar. Beat in orange juice, eggs and milk. Transfer this mixture to the prepared casserole dish.
In a small bowl, combine pecans, 1/3 cup brown sugar, flour and 2 tablespoons butter. Spread the mixture over the sweet potatoes. Cover the slow cooker and cook on HIGH for 3 to 4 hours.