Oh, I forgot to mention that we don’t consider Defender as other AV ;-). As it’s built in, it would mean we would be always in Passive mode after installation. But that’s not that case. And yes Defender detects that we are installed and runing and disables itself.
So, if you switch avast! to Passive Mode, Windows Defender kicks in or not?
WD is blocked in the latest versions anyway. 
It is however very easy to turn on the limited, on demand scanning:
http://screencast-o-matic.com/screenshots/u/Lh/1470660723161-13944.png
If you do it as a user manually, then yes. WD turns on. But that’s on WSC not on us.