Upon choosing the disable and unregister On-Access Scan with Windows Security Center option, the On-Access Scan gets disabled and unregistered from Windows Security Center and allows Defender to be set as the active antivirus alongside other Trellix defenses.
Task
Select Menu → Policy → Policy Catalog, then select Endpoint Security Threat Prevention from the Products list in the left pane.
From the Category list in the right pane, select On-Access Scan.
Click the Edit link for an editable policy.
Click Show Advanced.
Under On-Access Scan, select Disable and unregister with Windows Security Center to remove the on-access scanner from Windows Security Center and enable Defender as the active antivirus alongside Trellix defenses.
Click Save.