Preheat oven to 350 degrees. Spray two 8 inch round cake pans with non-stick spray.
In a large bowl, combine cake mix, egg whites, yogurt, and water. Beat until cake mix is smooth. Bake cakes for 30-35 minutes, or until a toothpick comes out clean. Let cakes cool to room temperature, but leave in cake pans.
In a small saucepan, combine strawberries, water, and sweetener, and bring to a boil. Reduce heat; simmer, uncovered for 3-5 minutes or until berries are soft enough to mash. Place strawberries in a strainer, and mash over a bowl, to remove strawberry pulp from strawberry sauce. Dissolve gelatin in strawberry syrup, and stir, (be sure to do this while it is still hot so the gelatin dissolves). Let cool to room temperature. Repeat with blueberry sauce.
Use a skewer to poke holes in the top of each cake layer, (cakes should still be in pans, and completely cooled at this point).
Pour cooled strawberry syrup over one cake layer, and the blueberry syrup over the next. Cover cake pans, and place cakes in fridge until gelatin is set, about 2 hours.
To make the frosting, whisk together the milk and pudding mix until smooth. Fold in cool whip, until smooth. Chill frosting in the fridge until the cake is completely cooled, and ready to be frosted.
Remove cake from cake pan by running a knife around the edge of the pans. (Run outside of pan under warm water if cake is difficult to remove. This will loosen the gelatin).
Remove the strawberry cake, and place on a cake stand, (or serving plate of choice). Spread 1 cup of the whipped topping over the cake. Top with the blueberry cake. Frost the cake with the remaining whipped cream.
Decorate with additional blueberries and strawberries if desired.
Chill for at least one hour before serving. Enjoy!