Can I use Managed AV (Microsoft Defender) alongside my current AV?

In short, "you can, but you shouldn't." 

By default in workstation versions of Windows (Pro, etc.), Microsoft Defender disables itself when a 3rd party AV is detected. 

Windows Servers do not automatically disable Defender when another real-time engine is present, but it's recommended that you disable Defender in scenarios to prevent conflicts. 

Please see Microsoft Defender Antivirus compatibility with other security products for more details.