Windows Defender gets disabled by the operating system (not avast) when you install any 3rd party antivirus.
Not sure why you would want to do this, if you installed Avast, because it is designed as a resident antivirus.
However, it is possible using the AvastUI > Menu > Settings > General > Troubleshooting > Enable Passive mode.
I don't know if this act will automatically set Defender as the Default AV, you would need to check that in the Windows Security > Virus and Threat Protection.