Use both boxes and prepare the cake mix as instructed by the box using ingredients from both boxes.
This will give you 2000 grams of batter.
Now divide the batter into 4 equal bowls (500 grams each to be precise)
You need only 3 bowls, you can use the other bowl for later.
In one bowl add red colour and in the other blue. One bowl should remain white.
Bake the cakes as instructed on the box.
Let the cake cool completely and refrigerate (covered)
While waiting for the cake to cool, make the butter creme frosting
in a blender or mixer creme the butter and shortening for two minutes until light and fluffy
add the vanilla and egg white and whip until fluffy
Gently add the sugar one cup at a time and mix until it well blended in the mixture.
stacking the cake
Using a cookie sheet or a cake leveller make sure all your cakes are all uniform in level
Now, cut the red cake in half horizontally. Carefully
Do the same for the white cake.
Stack half a red cake and half a white cake.
Now, use a bowl and cut out a center of the stacked cake.
Now, use the same bowl and cut out a center of the blue cake and place the red and white stacked center which you just cut out in the center of the blue ring. (Place the red/white circle inside of the blue ring.)
Now stack in this order : the other uncut half of the white cake, then the uncut half of the red cake and then blue ring with white and red cake in the center. Assemble each layer with butter creme frosting.
Coat the entire cake with frosting that will act as glue for the cake. Leave in the fridge for an hour.