I've recently been trying out https://www.spacedesk.net/
This allows you to use the screen of another laptops/PC/Tablet (Windows/Android/iOS or HTML5 browser) as an extended display.
I've given it a few goes and it's working much better than I expected it to.
It certainly works better/smoother if the main map is on the 'host' machine, but no real issues with windows that are mostly text being on the 'viewer' machine.
This image shows a clear main map whilst still having 6 other screens open.
1 'host' laptop + external monitor and 2x laptops running as 'viewer' for extended screens.
Only 1 mouse/keyboard gets used
I've not tried it with any really big scenarios yet, but small/medium ones run OK without any noticeable slowdown, even on a modest laptop. The only time I did see a problem was when the main laptop entered sleep mode.
This certainly is NOT a replacement for another monitor, but it's free (which is always good) and a does get around the issue of the number of external screens most laptops can run.
