So even more detailed. I played local media on my phone to a wiim mini and casted mixcloud mix to a wiim pro after that. But the local music apparently stopped which was in another room but same wifi
The WiiM Home App shows one "tile" per WiiM device and allows you to control them independently, of course. You need to make sure to select the appropriate tile on the "Devices" tab before streaming to the WiiM device in question.
So, generally speaking you can play totally different content to each WiiM device. Select the device on the devices tab, start music playback, select the other device, start music playback. For streaming services the WHA is just a kind of fancy remote. The music stream does not pass through your smartphone, but is requested directly by the WiiM device from the streaming server.
Limitations
may come into play when your smartphone is the actual data source.
Since Mixcloud is not a music service integrated into the WiiM Home App and this thread is tagged Chromecast I assume that you're using Google Cast to stream Mixcloud to the WiiM Pro. This
shouldn't affect the WiiM Home App streaming local files to the WiiM Mini at all.
To be on the safe side, you could pick the WiiM Pro tile before starting the Google Cast streaming, but this shouldn't be necessary, as you select the target WiiM for Mixcloud from its Google Cast target selector.
You can even play different local files to the different WiiM devices from the same instance of the WiiM Home App.