Setting Virtualbox "monitors number" to 2 - input locked

I am following this thread on how to open a Virtual box with two (virtual) monitors (=two windows, each correspondig to a monitor), so that I can display them on two physical monitors.

I am running Ubuntu on Virtualbox, on Windows 10.

I have my 2 physical monitors up and running (Windows settings: Display > multiple displays > extend these displays), I have my VM shut down.

Following step 4, I go to Virtualbox > My VM > settings > screen > number of monitors, but this parameter is locked, I cannot slide the bar.

Why is it locked? How can I unlock it?

Solution 1:

SOLVED

I was actually misinterpreting step 1:

I was shutting down my VM from Virtual Box

right click on the VM > shut down > save state

while I had to power off the VM from the VM window:

Ubuntu OS interface bar > power off/logout > power off...

Then, after my VM had rebooted, I had to go to

VM window > window upper bar > View

and here I can see two options: virtual display 1, virtual display 2.

Here I had to select

virtual display 2 > Enable

and another window opens, with a second screen.