Have you had a look at the AVAEMixerSample code at https://developer.apple.com/library/archive/samplecode/AVAEMixerSample/Introduction/Intro.html?
Specifically, around line 252 of https://developer.apple.com/library/archive/samplecode/AVAEMixerSample/Listings/AVAEMixerSample_AudioEngine_m.html
// fan out the sampler to mixer input 1 and distortion effect NSArray<AVAudioConnectionPoint *> *destinationNodes = [NSArray arrayWithObjects:[[AVAudioConnectionPoint alloc] initWithNode:_engine.mainMixerNode bus:1], [[AVAudioConnectionPoint alloc] initWithNode:_distortion bus:0], nil]; [_engine connect:_sampler toConnectionPoints:destinationNodes fromBus:0 format:stereoFormat];
I would try that method to do one of the following:
- Attach multiple mixers to the input node and attach the taps to those
- Attach a single mixer to the input node, and then attach additional mixer nodes to the single mixer, and install the taps on them
- Attach multiple AVAudioSinkNodes to the input node or an attached mixer (this is a new feature of iOS 13)