1. In a small pan, combine sugar and 1/2 cup water and bring to a boil. Boil until sugar has completely dissolved, about 5 minutes. Stir in lemon juice. Transfer to a bowl and set over an ice bath to cool to room temperature.
2. Use a chef's knife to peel grapefruits and oranges. Holding fruit over a large bowl, cut in between membranes and release fruit slices into bowl, catching any juice in the bowl as you go. Pour lemon syrup over fruit and stir until combined.
3. Using an electric mixer on medium-high speed, whip heavy cream and confectioners' sugar until soft peaks form.
4. In a large bowl, use hands to break up ladyfingers into roughly 1-inch pieces. Set up 10 8-oz. glasses. Divide 1/2 of ladyfingers among glasses. Top with 1/2 of citrus salad and 1/2 of whipped cream. Repeat, adding another layer of ladyfingers, citrus salad and whipped cream. Garnish each glass with a thin orange slice, if desired, and serve.