We have run some experiments on double side BGA assembly. Our preliminary results show that complete overlap of top and bottom BGA's indicate fair results, even after reliabiltiy tests (vibrations, temperature cycles, HALT). I will be interested to have feed-back from similar studies and get recommendations from others.
If at all possible avoid mirror assembly BGA designs. It has a higher failure rate when compared to non-overlap designs (life testing). In addition, from a rework point of view, X-ray inspection yields no help when a failure occures. You can't determin which device (mirrored) has the defect. Consequently, when you rework the top part the opposite part will reflow and lend itself to form solder "colums" and possible open connections.