How to add and remove synth in ofxTonic on the fly

Hey there
I am using ofxTonic and need to add/remove instruments respectively synth instances on the fly. eg each user entering a scene needs his instrument. after leaving i need to mute / delete it again.

i think, what confuses me is how to deal with that line:

void ofApp::audioRequested (float * output, int bufferSize, int nChannels){
    synth.fillBufferOfFloats(output, bufferSize, nChannels);
}

if i add more than one synth like that:

void MySynth::audioRequested (float * output, int bufferSize, int nChannels){
    synth.fillBufferOfFloats(output, bufferSize, nChannels);
    synth2.fillBufferOfFloats(output, bufferSize, nChannels);
}

i can only hear the last one.
Any hints?