Fill 8 (16 oz.) tumblers with ice. Pour 2 Tbsp. cranberry juice cocktail into each. Slowly add 2 Tbsp. orange juice to each to create another layer. Divide 1 liter orange-flavor seltzer evenly among glasses. Add 1 oz. rum to each glass, if desired. Shake 1 drop Angostura bitters on top of each. Serves 8.