This is not true for several reasons:

1) Software screen switching is not instantaneous

2) Moving your head is lower friction than pressing a button (and more likely a key chord)

3) Humans have very good spatial awareness and multiple screens have this affordance that a button would not.

Humans evolve. Computer systems aren't exactly "spatial", and virtually all attempts to model them as such have failed. We ought to strive for future vision.

1) False. Software screen switching is easily instantaneous. If you literally have two front buffers which, you must for a dual screen setup... there’s no reason you couldn’t flip between them at the refresh rate of your monitor (which is ridiculous but doable). Workspace switchers that run on shitty old X can be and many are effectively instantaneous. Not sure about Windows, but I assume something exists.

There’s more to life than Mac OS X.

3) you can still have a spatial layout of workspaces even with one viewport.

How come in people have been shown to be more productive with dual screens?

Well I didn’t say multiple screens didn't serve a purpose, did I?

Having said that, is there any good evidence of a fair comparison of dual screen vs a fast switching single screen. Most Mac and Windows setups make switching so tedious and slow it’s pretty obvious that dual would be more productive.

I would not be surprised that a truly friction minimized single screen setup (total switching latency from input to display <100ms... which is technically easily achievable (and done in many special purpose devices... just not in PC software).. and ergonomic switching key would approach dual screen in productivity. I mean, this is the typical setup I and many others use on a Linux laptop for dev work. By the time someone gets MC on OS X up, I can literally switch “screens” 3 or 4 times.

I enjoy a multimonitor (4) setup, but I’m not about to trade off battery life and space for it on the road.

Frankly it’s just easier and cheaper to buy a second monitor with state of the art, for most people. I get that. I use OSX and Windows too.

Also, I would expect more than 2 monitors to increase the gap.

People with multiscreen setups knows about alt-tab. It's about removing friction.

Virtual desktops were first envisioned at Xerox Parc and hot keys aren't new but dual screens have proven demonstrably more productive. Why is this?

Glancing from one screen to another is easier and lower mental overhead and you can look for input in your peripheral like text appearing without losing current context. You can also read from one screen while typing in another which is impossible in a non visible workspace.

This is how a robot thinks. It's much more intuitive to have two screens where you can see everything in your peripheral vision and dart your attention without any extra thinking.

In the case of the DS at least, the big differentiator is that the second screen is touch, and is positioned in a more touch-friendly place.

If your one-and-only screen is touch, you have to lift your arms up away from your buttons to press anything. No matter how much of an impost it is to have to look down a little, I'd say it's still less than having to lift your hand away from the buttons into your eyeline.