Sprinkle unflavored gelatin over 1/2 cup cold water and let stand 1 minute. Combine lemon-flavored gelatin, 1/2 cup boiling water and unflavored gelatin mixture, stirring until gelatin dissolves. Stir in sour cream, horseradish, and mayonnaise. Pour mixture into a lightly oiled 4 cup mold; cover and chill until firm. Yeild: about 4 cups.