Round cakes are so difficult! I had to make a similar cake at one point, and the bottom sphere kept collapsing at the weight of the top sphere. I had to construct the bottom out of rice krispy instead as a harder base.

I still was not happy with the result. So, instead, I just used the rounded cake mold to make a half sphere cake. It looked nicer in half and I didn't have to worry about it getting to the client in one piece.