Linux client can't use second monitor via Remote Desktop

[nubaltec]

I have the following setup, which I give in full here in case it has some influence in the problem: The physical workstation "One" is a Devuan/XFCE based Laptop with two external 1920x1080 monitors connected. The physical workstation "Two" is a Windows 10 box somewhere far away.

On "One" I have a virtualbox 6.0.14 with a Windows 10 guest "v1" running. "v1" uses the two external monitors in full screen display and connects via VPN and Remote Desktop Connection to "Two".

On "Two" I have a virtualbox 6.0.14 with a Devuan XFCE MV "v2" running. "v2 is supposed to use both displays of the remote desktop connection, which are rendered on the two external monitors of "One".

What happens is that I do have both full screen displays of "Two" on the external monitors of "One" used by "v1", but "v2" only ever opens monitor 1. I did manage to get a small window for the second display once or twice by playing around with the view parameters for a long time, but as soon as I try to maximize it the window folds to never appear again. The "Display" settings inside "v2" only show me "Virtual1" monitor available.

I have attached the two screenshots made from "One". "screen1" shows the display of "v2" inside the remote desktop from "v1" to "Two", so on top there is the menubar from "v1" (in Spanish), at the bottom the menu bar from "v2" (in German). Please ignore the content of the target desktop. "screen2" shows the empty second monitor from "Two" via remote desktop from "v1", so it only has the "v1" menu bar on top and the windows background of "Two". What I expected to see there was the second full screen display of "v2", because that is the machine I di the actual work on, so I need both displays there.

Log from v2 attached, as soon as I find out how attaching files work.

[nubaltec]

Logfile of the target virtual machine "v2"

[nubaltec]

screenshot of monitor1 with menu bars from "v1" and remote desktop above and "v2" below with dummy content filler

[nubaltec]

screenshot of the other monitor, vm menu only has one window

[nubaltec]

I also discovered yesterday that the keyboard bounces in this configuration, i.e. while editing source files in IntelliJ IDE keypresses are repeated which leads to errors in the edited file. Not sure which part causes this.