I have linux on host and guest (Debian & Debian).
Virtualbox 7.0.8 with the Guest Additions.
Your VM has to be configured with 2 monitors, also the Host.
I got it working, but it is a bit tricky. Sometimes you have to try several times, but it works (at least with my config).
Get the two windows representing each monitor, maximized in each monitor.
Set "Auto-resize Guest Display"
Set Input > Mouse Integration (without this, I got the screens working well, but the mouse was moving on both screens at the same time).
[Correct me if I am wrong] For "Mouse integration" to be there, the VM has to be started in 'Normal' mode (not Detached mode).
Now get it to Fullscreen.
You may notice that the screen on the right OR second screen is not visible, while the first one is full screen in its own monitor. Sometimes, it is there, but at the bottom, where only the Virtualbox menu for the VM is visible when you move the mouse to the bottom. In the menu, click on Maximize. Other times, the 2nd screen is there (because of the VirtualBox menu) but got Disabled and you have to Enable it again).
As mentioned above, you may have to play/cycle a few times, until it works.
Note: My 2 monitors have different sizes and it works well, as far as the resolution on the Host machine is properly setup.