Can I make two monitors into one screen so that an app would go fullscreen on both of the monitors?

I have two of my external displays put vertically next to each other. I would like them to be treated as one monitor/screen so that when I maximize a window, it would go over both of them. This is something that should be possible: https://bbs.archlinux.org/viewtopic.php?pid=1641219#p1641219 but nvidia drivers do not seem to support xrandr --addmode

Is there any other way to achieve this?

In theory, that should be a use-case for xrandr --setmonitor, i.e. randr 1.5 monitor objects, and at least gnome should honor that. I had no success when playing with that some time ago, though. Alternatives might be fakexrandr or fakexinerama.

And is there any indication that nvidia driver supports --setmonitor command when it refuses --addmode?

Anyway, I found out this seems to work: https://askubuntu.com/questions/150066/split-monitor-in-two

i.e. tell compiz not to detect displays. But it seems now my monitors go blank every 15 seconds, hm!

Anyway, the script for my monitors seems to be:

#!/bin/bash
state=gsettings get org.compiz.core:/org/compiz/profiles/unity/plugins/core/ detect-outputs
if [ $state = “true” ]
then
gsettings set org.compiz.core:/org/compiz/profiles/unity/plugins/core/ detect-outputs false
gsettings set org.compiz.core:/org/compiz/profiles/unity/plugins/core/ outputs “[‘2560x1440+0+0’, ‘2880x2560+2560+0’]”
else
gsettings set org.compiz.core:/org/compiz/profiles/unity/plugins/core/ detect-outputs true
fi

(since when I remove the laptop from the dock, I need Compiz to start detecting those monitors otherwise it still thinks I have three external ones)

addmode is just refused if the output (the monitor) does not support that mode.
The VIRTUAL1 output is a special feature of the old “intel” driver, no other driver supports that.