You want to know the expected behavior when run manual scan during scheduled or manual scan running.
Symantec Endpoint Protection for Linux
Symantec Endpoint Protection (SEP) is designed to run one scan only at a time. If you run manual scan during the scheduled or manual scan running, manual scan is queued and will start after the scheduled scan is completed.
On Windows SEP client, if you run manual scan during scheduled or manual scan running, the following dialog will be displayed and manual scan will be queued.
"Another scan is in progress and will be queued if you want to continue scanning"
On SEP for Linux client, manual scan will not be queued, it is not same as Windows client.
If manual scan is executed during scheduled scan running on SEP for Linux client, errors such as "error writing to client" and "error reading socket" may be logged in /var/log/messages due to the timing issue. In that case, restart SEP for Linux daemons by following procedure.
How to restart the Endpoint Protection Linux daemons