Boil the Jello, sugar, and water (3 cups) stirring constantly.
Dissolve the Knox Unflavored Gelatin by stirring in the cool water (2-1/2 cups) and set aside.
After boiling the lime Jello, combine it with the Knox Unflavored Gelatin mixture and pour into a greased (spray with vegetable oil) 9-inch by 13-inch pan.
Set in the refrigerator until it sets up and is ready to cut into cubes, or use cookie cutters.