Mixer C on the synthesizer can be controlled using the midi commander (only for Android) Connect the tablet to the synthesizer, in the settings of the Thru commander and midi commands from mixer A will go to mixer C, and from the commander you can already turn on the desired instrumental sounds on mixer C, and also control all synthesizer controllers. In this way, you can solve the problem by adjusting the volume level of two tones. In this case, the first tone will be on mixer A, and the second on mixer C and the second tone will be completely under your control through the tablet. And if we also turn on the Layer on the synthesizer, we will get four tones in total, but it will probably be all too difficult to set up for a start, although I use it, that is, I play four layer tones on the CASIO CT X 3000. Link to the https midi commander : //play.google.com/store/apps/details? id = it.bordero.midicontroller & hl = en
Use it, it's free.