I agree, I alwayscook the top and bottom separate, as the top takes less time to cook.
The first time I ever did one, I made the recipe from scratch and it was a disaster. Such a waste of ingredients! So after that I cheated and just used cake mixes .... I think I ended up using about 4 boxes all up. Its better to have the mixture overflow slightly than to not rise enough, you can always level off the top of the cake, but if it doesnt rise enough because there isn't enough mixture, then the top doesnt look right on the base.
I make these for DH who's done quite a few cake smashes for his photography business. I always cook them much slower than the box suggests, otherwise they burn on top.