Quote:
Windows Defender won't run in the presence of most reputable antivirus products, since once they register themselves as an AV provider Defender will automatically turn itself off excatly as it's designed to do..
Microsoft has never ever supported the simultaneous operation of 2 true AV applications at the same time, since the design of the core Windows API calls that support their operation is only capable of managing one security app.
Though there are other security apps like Malwarebytes Anti-Malware (MBAM) that do claim to support simultaneous operation with Defender, that's not supported by Microsoft itself, so it's up to the Malwarebytes to ensure this continues to work and we've seen a few issues with this in recent times as well as the past. So generally those here don't recommend this and instead suggest using MBAM as on on demand scanner.