Mix plums, cherries, sugar, and allspice in heavy large saucepan. Cover and cook over low heat until sugar dissolves and juices form, stirring occasionally, about 15 minutes. Uncover and increase heat to medium. Cook until plums start to fall apart and preserves are thickened and reduced to 2 1/4 cups, stirring occasionally, about 25 minutes longer. If desired, break up any large plum pieces. Transfer to bowl and refrigerate until cold, about 2 hours.
