How to try what I'm doing... Try it, it's fun, it's only 1+n commands. No root, reboot or no config files needed.
My laptop screen is 1366x768 so here are the commands I'm using.

apt-get update && apt-get install x11vnc

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Get a VNC viewer, on linux try vinagre.
On Android, if you search for "RealVNC" You'll find "VNC Viewer" which has RealVNC's logo.
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#first I adjust the framebuffer to be 3x the width of my laptop screen
xrandr --fb 4098x768 --output LVDS1 --panning 1366x768+0+0/4098x768+0+0
#at first I was going to match the max resolutions of my phones, but 1920x1080 on a mobile phone screen at arms length is a bit tiny, so it's pointless. I'd rather run a lower res and let it be zoomed.

#server/phone A (works on it's own)
x11vnc -clip 1366x768+1367+0 -rfbport 5901 -viewonly

#server/phone B (works on it's own)
x11vnc -clip 1366x768+2734+0 -rfbport 5902 -viewonly

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If you don't have two phones, you can just use one or two vinagre tabs or whatever vnc viewer you like.

If I connect either phone independently, it works fine.
If I connect them both simultaneously, when the 2nd phone connects I get *** buffer overflow detected ***: x11vnc terminated

The x11vnc instances are conflicging with eachother somehow. Is there a way to make each server run in it's own namespace?

Do you know what's wrong?
I've looked through the manual, but it's a monster, and I didn't find anything.

I saw the environment variable VNC_CONNECT listed, but it doesn't seem to be applicable to this use case. Anyway, I tried this
`VNC_CONNECT=foo x11vnc -clip 1366x768+1367+0 -rfbport 5901 -viewonly` and it did not help anything.

EDIT
Okay I got it working.
I just made a single server double the width.
`x11vnc -clip 2732x768+1367+0 -rfbport 5901 -viewonly -forever -shared -many`
Connect both phones to the same server, pan/zoom each one into position.

CONCLUSION
Cool party trick. But the performance sucks too bad to be of any use. I can't see the mouse pointer move on the phones. It just stays in some arb position. Updates are quite slow (measured in seconds).

You could perhaps put a bit of documentation on the mobile screens, but positioning windows on them is a hassle, as is the whole connection/panning/zooming procedure.

It's not worth it.

If anyone finds a way to make this suck less, let me know.

Android phones vary hugely in performance & specs. Many (like mine) is on an arm 7 & Mali 400 gpu, which is not enough. Then apps run Java, when you want to issue OpenGL commands - work for the underspec'ed CPU. You can't even get a flash plugin for it. I went looking with an x86 Intel Atom on Android, and the website laughed at me. "Get a CPU, sonny before we can do a plugin!"