Roll chilled pastry to 1/8-inch thickness on a lightly floured surface. Place in a 10-inch deep dish pie plate; trim excess pastry. Fold edges under, and flute. Set aside.
Combine sugar, syrup, eggs, spices, and salt in a large mixing bowl, beating well. Stir in sweet potatoes, pumpkin, rum, vanilla, and milk.
Pour pumpkin mixture into prepared pastry shell. Bake at 350° for 1 hour and 15 minutes or until set. Cool pie to room temperature.
Beat whipping cream until foamy; gradually add sugar, beating until soft peaks form. Garnish with whipped cream; sprinkle with additional cinnamon.