It felt simultaneous to me (an outside observer) but it was probably more like fast switching, with the ability to keep a (probably large) set of chunked data associated with each context. I've seen him rapidly switching between writing code on one box, debugging code on another, maintaining separate (different and quite technical) conversations with three (including me) people in his office, plus another via speakerphone.

He was an... unusual individual. I'm not saying that 'just anyone' can do that. I certainly can't. If you think nobody can, I have a black swan for you. :-)

Oh, and yeah, that bit about autism was definitely speculation. But a lot of the people I worked with were borderline on the spectrum - so it's more from personal experience and reasoning that if both parents were like that, you can only rely on reversion to the mean so far. I'm fine with being wrong.