1. Drain cherries and pineapple, reserving 1 1/2 cups juice in a saucepan. (If necessary, add water to equal 1 1/2 cups.) Bring juice mixture to a boil over medium heat; stir in gelatin, and cook, stirring constantly, 2 minutes or until gelatin dissolves. Remove from heat, and stir in 1 cup cold water. Chill until consistency of unbeaten egg whites (about 1 1/2 hours).
2. Gently stir in drained cherries and pineapple. Pour mixture into a 8-inch square baking dish or 8 (2/3-cup) molds. Cover and chill 8 hours or until firm. Dollop with mayonnaise, and garnish, if desired.