That kind of "Mixer" doesn't seem to be possible. Quoting from the description of App Volume (thanks to Fiksdal for pointing to this comment):
Android manages one stream for all apps, so we can't set different volume levels at same time for two different apps.
So at the very best, you could set different volumes for different apps. Again, there's nothing built-in to Android to automate that, so all solutions are app-based:
- App Volume: Claims to cover exactly your use-case – but was last updated 11/2013, so it's not clear whether it will support recent versions of Android.
- AppConfig: can do this at least for media volume (and some other settings). Again, not updated for a while (since 6/2014).
- App Volume Manager: again an app last updated 10/2014, and not having the best rating – but according to this XDA thread, it should do the job.
A mixer could be possible in a limited way if you could tell the involved apps to use different "channels" (e.g. one using the ringer, one multimedia, one notifications, one alarms) – which might be possible for a root app (with some XPosed module being the most likely candidate). But so far, it seems nobody came up with such.
I don't know about any more up-to-date candidate or other alternative, though some might have slipped through my nets. Be welcome to check my app-list for System Settings: if I happen to find one, it will certainly be added there.