To make the crust: cream together the butter, brown sugar, flour, and chopped walnuts. Press crust down into a 9x13 glass dish. Bake at 375 degrees F for 10 minutes. Set aside to cool.
Dissolve box of Lemon Jell-O in 1 cup boiling water. Set aside to cool. In large bowl, mix sugar, Philadelphia Cream Cheese, and Cool Whip with electric mixer. Add Lemon Jell-O and mix until blended. Pour over crust. Refrigerate 1-2 hours.
Dissolve box of Lime Jell-O in 3 1/2 cups boiling water. Set aside or refrigerate for 10-15 minutes to cool, but not long enough to set. Pour Lime Jell-O mix on top of cream cheese layer and refrigerate until Lime Jell-O layer has set.