Hard to tell what's going on from your description, but I wonder, if the adhesion between layers IS an issue, whether you could rip up just one layer. Once you begin the tearout, it should be fairly obvious whether you can.

You might want to post your question in the john bridge tile forums.

Please post what you learn there back over here as this will be of interest to many.

We have two areas in our house where there is tile over tile in place for 4+ years, and neither area has evidenced any problems. However, it was a lot of work to do correctly. (It would have been more to remove the tile, which we did try first, because it was STUCK!) In both cases, the lower tile floor was in absolutely perfect structural shape -- no cracks or hollow-sounding spots. And before going over the top, we roughened up the surface of the lower layer to assure good adhesion.

Sewhappy - It sounds to me like your seller did a 'quick and dirty' remodel for sale, and probably skimped on the floor preparation since it wouldn't show. Wouldn't show prior to the sale, anyway... It might be worth a little research to see if you have any recourse.

Sorry, but I think you have to tear it out and start over. We took out our kitchen tile and found live electrical boxes buried under the tile -- go figure. At least you --hopefully -- won't have that to deal with!