Or it's like arguing that no one could run Apple and Pixar or SpaceX and Tesla.

Jobs ran Apple. He didn't run Pixar. He had a similar title at Pixar, but his role was completely different.

Agreed, as a shareholder I'd also prefer to have a well rested CEO dedicated to just one company

Many employers have agreements prohibiting their employees from having other jobs. Most people can't work two full-time jobs productively.

How many hours would they expect him to put in normally?

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.