Combine the reserved pineapple and mandarin-orange juices.
Add enough water to make 2 cups liquid.
Bring the liquid to a boil.
Remove from the heat and add the orange sherbet.
Stir until the sherbet melts.
Add the orange gelatin and stir until completely dissolved.
Stir in the crushed pineapple and mandarin oranges.
Pour into an approximately 8 × 10-inch glass dish.
Refrigerate until firm, preferably overnight.