unhappy switching operators. 2 men required to remove (rack out) 1 breaker for safety, if we are talking ~5kV / ~15 kV breakers. For low voltage it is not uncommon to have up to 4 480V breakers high.
The bus will be in the middle, so on the top breaker the feed is from the top poles of breaker. On the bottom breaker it is from the bottom.
This can cause issues with training and labeling the ground and test breaker.
The space savings are obvious. If there is not much racking in / out probably not much an issue. My local utility does not like them for the above reasons.
Consider an electric racking (and lifting) device if you do go 2 high. It makes life easier (and cheaper) for the maintenance.
I haven't heard anything one way or the other, but I suppose if you had a catastrophic failure in a lower breaker, the upper one would suffer (presumably) damage as well.
