Have second user in window and not whole desktop
OK - I have found a solution using Real-vnc. 


Install Real-vnc version 5.3.2 from their download site. It is important to not install a newer version, as this seems to be the last version which supports User-mode for the server without a paid subscription!
As the server stops when a fast user switch occurs it is important to change the following setting (from the real-vnc knowledge base):



  ... setting the option "StopUserModeOnSwitchOut" to "No" in the Expert tab
  (VNC Server > Options > Expert tab).



Quit VNC server (this is the service). We don't need the global VNC server service as we will use the User Mode.
Now switch to the other user and login
Start VNC server (user mode). If you want to, you can add it to the Login Items to have it autostart upon login.
Switch back to your main user
use Screen Sharing app or the VNC Viewer app to connect to localhost. It is possible that you have to set a password in the VNC server, but I am not sure (the viewer will tell you).


That's it - it works perfectly for me, even with multiple monitor setup.

What I didn't manage, was to setup that the viewer uses both monitors - but I can live with that.