On Windows 10 it is Windows that disables Defender when another av is installed.
On versions prior to Windows 10 it is avast who is doing so.

You can run Windows Defender while having avast installed.
To do so, tell avast to run in “quiet mode” (If I remember correctly that is how it is called)
Defender will run in real time and avast can be used as a on demand scanner that way.