So I finally figured this out. Vino used to be the default VNC server in Ubuntu, but that is now deprecated and replaced with "gnome-remote-desktop" according to
https://wiki.gnome.org/Projects/Vino
And there is an open bug with gnome-remote-desktop "Support multiple simultaneous VNC clients to connect to a session", i.e. this new VNC server does not support what vino did:
Disabling remote desktop sharing in Zorin settings and installing and configuring vino restores the ability to access your Zorin desktop from multiple clients simultaneously. My use case for this is I use Zorin as a dashboard for home automation displayed remotely on a ChromeBit (using Remina), and I also need to VNC from other computers to my Zorin box to edit/update this dashboard.
Here are the steps I took to install vino:
sudo apt install vino
gsettings set org.gnome.Vino prompt-enabled false
gsettings set org.gnome.Vino authentication-methods "['vnc']"
gsettings set org.gnome.Vino vnc-password $(echo -n "mypassword"|base64)
gsettings set org.gnome.Vino require-encryption false
Your settings may differ, you can list all Vino settings using:
gsettings list-recursively org.gnome.Vino
To run the server use /usr/lib/vino/vino-server
To run this on startup, add this command to list of Startup programs in Startup Application Preferences GUI.