I don't think it would be the hours as much as the mental focus. When I get done working for 8 hours, I'm usually with my family about an hour and a half before I can mentally break away from what I've been doing all day, and some nights my mind will always be on my tasks no matter how hard I try to refocus.
Imagine going from 1 high pressure 8 hours job to another with only a 2 block walk between them, maybe lunch. I don't know, it sounds like overload to me.
I find it is significantly easier to work two eight hour jobs than it is to work one sixteen hour job. The context switch is refreshing.
Unlike Dorsey, I have never attempted to maintain that schedule over the long haul, but over periods of time at least, it is not as difficult as it sounds.
Imagine going from 1 high pressure 8 hours job to another with only a 2 block walk between them, maybe lunch. I don't know, it sounds like overload to me.