Carefully wash the oranges and lemons. Dry them and cut off their tops. Using a small spoon, carve them and take out the pulp. Pour the sugar in 2/3 cup (150 ml) water and stir until dissolved. Boil this sugar syrup.
Remove the syrup from the heat and leave to cool. Pass the pulp through a sieve (do not mix the 2 types). Divide the syrup in 2 parts and mix it with the orange and lemon juice.
Beat the whites of the eggs into thick snow. Again, divide them in half and distribute them among the orange and lemon mixtures, stirring carefully.
Place in the refrigerator for 3-4 hours, while stirring with a fork every 20-30 minutes. The finished frozen sorbet can be served in a glass or returned to the hollowed out fruits, for a more unique presentation.