The issue is that due to the way AV operates, you would typically have only one "active scanning" program active at one time. If you install Avast, it should automatically become your active scanning program and replace Windows Defender -- although you can still use Defender for on-demand scans if you like.
With
Avast, you get a lot more than just the basics (check out
CyberCapture and
Behavior Shield for some more info on our nextgen shields)!
Thanks for your question and for your support, we hope this info is helpful to you!