problem with high sierra multiple users simultaneously
First item, you get two sessions max. If you are struggling with the first or second session, then the issue could be your VNC software that connects to the mac isn’t following Apple’s conventions to allow the second session to start. Since you didn’t specify versions of software you use on the connecting side, I’ll guess and give general advice.

There is no change I’ve needed on the mac that’s sharing the screen, just your client needs to be updated or you need a different client, but some people from Windows report you need specific settings.


Screens iOS app works to open a second session
Screen Sharing app on the Mac (connect to server Finder menu - use vnc://whatever to connect) and Screens app on macOS all work with multiple sessions.


Apple Remote Desktop doesn’t open a second screen by default. It might have that option, but I only use it in the way you don’t - to share one session together.

Based on what os you are connecting from - these might help:


How to use Mac OS X &quot;Per-user screen sharing&quot; from a Windows PC?
Force screensharing to open new session
Are the changes to Lion&#39;s screen sharing documented anywhere publicly?