Hmmm.....Well, that's what I did. 

My first group was composed of only positive and negative cubes.  I collapsed that.  My second group was composed of a pos sphere and some neg cubes.  I collapsed that too.  I made the second object negative, the first positive and grouped them.  It didn't work. :o(

I've since found a way to get the effect I wanted a different way but I'd still like this method to work.  I'll have to play with it some more it looks like. :o)

Thanks!