I need to make a 2 tier cake with rkt "softballs"  in between, but I can't figure out how to stack it all. Can anyone help me?

You put columns/large dowels/SPS legs in the middle of the softballs.

Or, more correctly, you create airspace with either tall SPS legs/dowels/wilton legs or wilton two plate system.  Thew softballs cover the legs.  Make the softball then hollow out a space for the support piece.