Combine 1st package of Jello with 1 ½ cups boiling water. Stir until dissolved. Refrigerate until starting to thicken, about 60-90 minutes, watching carefully to ensure it doesn't start to set.
Drain horseradish in coffee filter or paper lined strainer.
Pour ½ cup whipping cream into large bowl and whip until stiff. Add mayonnaise and drained horseradish.
When Jello is cooled and thickening, fold into cream mixture and blend until well combined. Pour into 9 x 9 pan. Refrigerate until set, about 30 minutes.
While bottom layer is setting, prepare second box of Jello as per regular instructions. Refrigerate until cool but not set, about 30 minutes or less.
Pour Jello carefully over chilled bottom layer. Refrigerate until totally set. At least 30 minutes.
Notes
The original recipe calls for lemon or pineapple Jello for the top layer.
Can be made ahead and will keep in refrigerator for a few days.