The A/B switch would have to flip all 20 or so connectors from one to the other.
Now having three cables makes more sense. (I guess I didn't understand what you were saying before...) The switch is literally a switch in the simplest form, with nothing else going on. That might be nice, but it seems like a major waste to me. The small HDMI display backpack might be a little more useful to have a dual screen palmtop.
As the sun has started to come out a little more frequently, and the temperatures more agreeable to working outside, I've started to wonder if there's any linux device I could buy with an e-ink screen and proper keyboard. Perhaps the Pyra's screen will be bright and contrasty enough, but based on my experience with my Pandora I'm not convinced yet. I don't really need much; just a terminal and a headphone port should enable me to do some coding while listening to recorded music or radio programmes.
Kind of building on my stupid idea, would a standalone display attached to your Pyra work for you? It should also increase your battery life off used instead of the main display, at the expense of colour and touch input. I might even consider taping one to the back of my Pyra so I don't need to open it to check messages or who is calling. I could also use it for email and other text heavy stuff, but I would have to retape it on the inside. Actually, I am starting to like this idea. Now I just need to find one that is durable, affordable, and thin. Damn you, levi!
Other than listening to stuff, what else do you actually need for what you are planning on working on? I might have some suggestions that don't exactly fit what you are looking for, but they are still really useful, fun, and cool (in roughly that order, although how fun and cool depends on the individual).
On second thought, the displays may heat up with a lot of exposure to Sun, so maybe not that great for you. Why can't you just do your programming inside, like a normal person?