Put the cherries and sugar in a medium saucepan with 1/2 cup of water. Bring to a boil, reduce heat, and simmer for 5 minutes. Drain and place in a medium bowl. With a melon baller scoop the flesh from the melons into balls and mix the melon balls with the cherries. Reserve the melon shells. To serve, scoop the vanilla and pistachio ice cream into balls and fill the reserved melon shells with the ice cream and fruit. Serve immediately.
