That's entirely down to configuration on the target. Guacamole is just an in browser client, so it depends how the server is set up. For VNC, if your server is

That's entirely down to configuration on the target. Guacamole is just an in browser client, so it depends how the server is set up. For VNC, if your server is set up using X11VNC then you will be logging into the existing session, whereas if you set it up it TightVNC or Vnc4Server (can't remember what's in the Ubuntu repos) then it will be a separate session.