Prepare cake and bake according to package directions in a 9x13 inch baking dish. With a fork, poke holes all over top of cake.
Combine 1 cup hot water and 1 cup cold water with one package of lemon gelatin. Stir until gelatin is dissolved, and pour mixture over cake. Chill in refrigerator until cool.
In large bowl stir together milk, vanilla pudding mix and remaining package of lemon gelatin until powders are dissolved. Fold in whipped topping and spread mixture over cake. Refrigerate until serving.