I have made a TON of topsy turvy cakes and the only time I've had ANY problems was when there wasn't enough difference between the sizes of the tiers. I tell people there has to be at least 3" difference.......I prefer 4"+ size difference personally.....this way you guarantee you have enough of a ledge and the cake doesn't want to just break way on the sides.
Another important thing is (if you do the carve a hole in the bottom cake, insert top cake method) make sure that your hole is big enough that the top cake literally isn't touching any part of the hole, just enough of a gap between the 2 cakes that there is NO pressure on that bottom cake from the top cake, THAT will cause problems too. 9 times out of 10 people like the big funky fondant ball borders which are EASY to hide the gap with. Otherwise, once you stack you can pipe a little extra icing to fill the gap and smooth it into the rest of the cake.
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