Well, I have no experience with VNC and even less so with XFCE, so I don’t know whether either of them supports multiple concurrent logins from the same user.
The operating system itself and the X11 display server definitely do both support it — GNU/Linux is a UNIX/POSIX operating system, after all.
Thanks. And nice to have my own observations confirmed by “official” guidance.
Only, the same user account can have simultaneous local and TigerVNC X sessions in Ubuntu (and get two distinct desktops or even desktop environments). Therefore what we found would seem to be a limitation of Arch or Manjaro.
I only add this last bit so a random person landing here does not get thrown off. Perhaps they have Ubuntu.