Immersed is an application that allows you to connect to your computer from within Virtual Reality and use the full power of your Windows/Mac/Linux machine to run apps etc. but using virtual screens that are in a VR world. This can be useful if you want to get away from the home office environment and into a world where you can fully concentrate on the task at hand, or if you're travelling and can't take around your usual multi-screen physical setup. The Windows and Mac versions of Immersed allow you to create additional "virtual" monitors so that when you enter into VR you can see not only your physical display but one or more "virtual" displays too. Unfortunately the Linux version of the desktop agent does not yet support this feature, so if you have a 2 screen setup in the real world, then you'll see 2 screens in VR. However, there is a way of adding these screens in Linux using a tool called "xrandr" (works with X11). I originally tried the methods documented on the virtual-display-linux GitHub page, however this doesn't work when using Cinnamon desktop on Linux Mint as the additional virtual screen causes Cinnamon to crash on startup. It actually turns out that you don't need to create a virtual screen as long as you have some unused graphics ports on your machine, for example on my machine running xrandr -q shows: 
Screen 0: minimum 320 x 200, current 3200 x 1200, maximum 16384 x 16384
eDP-1 connected (normal left inverted right x axis y axis)
   1920x1080     60.02 +  60.01    59.97    59.96    59.93    48.00  
   ...
DP-1 disconnected (normal left inverted right x axis y axis)
HDMI-1 disconnected (normal left inverted right x axis y axis)
DP-2 disconnected (normal left inverted right x axis y axis)
HDMI-2 disconnected (normal left inverted right x axis y axis)
DP-3 disconnected (normal left inverted right x axis y axis)
HDMI-3 disconnected (normal left inverted right x axis y axis)
DP-3-1 disconnected (normal left inverted right x axis y axis)
DP-3-2 connected primary 1600x1200+0+0 (normal left inverted right x axis y axis) 367mm x 275mm
   1600x1200     60.00*+
   ...
DP-3-3 connected 1600x1200+1600+0 (normal left inverted right x axis y axis) 367mm x 275mm
   1600x1200     60.00*+
   ...
The "eDP-1" adapter is my physical laptop screen (which is connected but with the lid closed is not active). Then my two physical monitors plugged into my docking station are both running under the "DP-3" display port, as "DP-3-2" and "DP-3-3".. This means I have "HDMI-1", "HDMI-2", "DP-3-1" all available to "plug something in". You don't actually need to physically plug something in to use these in VR though, so I can just activate one (or more) of them at my desired resolution and position it wherever I'd like it to appear when I enter VR. In my case I like to split my two physical monitors apart with a 3rd low res, wide monitor (1400x900) that makes working in VR easier. (This was on the Quest 2 headset, see update below for higher resolution headsets). For example: 
Real world:
/--------\ /--------\
| DP-3-2 | | DP-3-3 |
\--------/ \--------/

Virtual world:
/--------\ /--------\ /--------\
| DP-3-2 | | HDMI-1 | | DP-3-3 |
\--------/ \--------/ \--------/
To achieve this, I've written a shell script which will add the new display settings before starting up the Immersed Agent, and will then reset the settings when the process finishes: 

#!/bin/sh
xrandr --addmode HDMI-1 1400x900
xrandr --output DP-3-2 --pos 0x0 --output HDMI-1 --mode 1400x900 --right-of DP-3-2 --output DP-3-3 --right-of HDMI-1
~/.local/bin/Immersed/Immersed-x86_64.AppImage
xrandr --output HDMI-1 --off --output DP-3-3 --right-of DP-3-2
Update 2023 Since upgrading to the Meta Quest 3 I can now work in 1920x1080 on all my monitors as they're clearly visible even at this resolution. This means I no longer need to put my "low res" monitor in the middle and I actually prefer to keep my "primary" monitor in the middle (I've also switched away from using HDMI port as DP-3-1 was available). So the new configuration looks like the below: 
Real world:
/--------\ /--------\
| DP-3-2 | | DP-3-3 |
\--------/ \--------/

Virtual world:
/--------\ /--------\ /--------\
| DP-3-1 | | DP-3-2 | | DP-3-3 |
\--------/ \--------/ \--------/
See updated bash script below: 

#!/bin/sh
xrandr --addmode DP-3-1 1920x1080
xrandr --output DP-3-1 --mode 1920x1080 --left-of DP-3-2
~/.local/bin/Immersed/Immersed-x86_64.AppImage
xrandr --output DP-3-1 --off