The Indigo UI app controls starting the Indigo Server. You only want to press the Start Server or choose the Indigo->Start Local Server... menu item from 1 macOS user account. Pick which account you want to start the Indigo Server and that should be the only account you ever choose that menu item or press the Start Server button if the dialog happens to show. On your other macOS user account you will choose the Indigo->Connect to Remote Server... menu item and enter the address 127.0.0.1 or localhost.
Whenever the Indigo UI app is launched it will remember which "mode" it is in based on what you last did. If you chose Start Server it will try to start up a new server (and then automatically connect to it), whereas if you chose Connect to Remote Server it will just connect to the already running Indigo Server (it must of course have already been started, in your case by the other macOS user). That is, in the latter case it won't start the server but in the former it will.
I'm fixing the crash that occurs when a 2nd Indigo Server instance tries to start for the next Indigo feature release, but note the root problem here is that you have multiple macOS user accounts trying to start the Indigo Server.
Lastly note you may need to restart your Mac after stopping the Indigo Server under the other account to get the repeated crashes to stop.