How do I output audio from multiple devices in CoreAudio?

Audio Midi Setup allows a single sound source to be output from multiple devices, but is there a way to do this in CoreAudio?

Could this be done by attaching the same playerNode to two AVAudioEngine? I thought about doing this, but it threw a stack trace and gave me an error.

import Cocoa
import CoreAudio

@main
class AppDelegate: NSObject, NSApplicationDelegate {

    @IBOutlet var window: NSWindow!


    func applicationDidFinishLaunching(_ aNotification: Notification) {
        // Get the default output device
        var defaultOutputDeviceID = AudioDeviceID(0)
        var defaultOutputDeviceIDSize = UInt32(MemoryLayout<AudioDeviceID>.size)
        var property: AudioObjectPropertyAddress = AudioObjectPropertyAddress(mSelector:kAudioHardwarePropertyDefaultOutputDevice, mScope: kAudioDevicePropertyScopeOutput, mElement: kAudioObjectPropertyElementMain)
        var getDefaultOutputDeviceIDStatus = AudioObjectGetPropertyData(AudioObjectID(kAudioObjectSystemObject), &property, 0, nil, &defaultOutputDeviceIDSize, &defaultOutputDeviceID)
        
        // Get the number of output devices
        var outputDeviceCount = UInt32(0)
        var outputDeviceCountSize = UInt32(MemoryLayout<UInt32>.size)
        property.mSelector = kAudioHardwarePropertyDevices
        
        var getOutputDeviceCountStatus = AudioObjectGetPropertyData(AudioObjectID(kAudioObjectSystemObject), &property, 0, nil, &outputDeviceCountSize, &outputDeviceCount)
        
        // Get the IDs of all output devices
        var outputDeviceIDs = [AudioDeviceID](repeating: 0, count: Int(outputDeviceCount))
        var outputDeviceIDsSize = UInt32(MemoryLayout<AudioDeviceID>.size * Int(outputDeviceCount))
        property.mSelector = kAudioHardwarePropertyDevices
        var getOutputDeviceIDsStatus = AudioObjectGetPropertyData(AudioObjectID(kAudioObjectSystemObject), &property, 0, nil, &outputDeviceIDsSize, &outputDeviceIDs)

        // Set the output device IDs to the first two devices
        var outputDeviceIDsToSet = [outputDeviceIDs[0], outputDeviceIDs[1]]
        var outputDeviceIDsToSetSize = UInt32(MemoryLayout<AudioDeviceID>.size * outputDeviceIDsToSet.count)
        property.mSelector = kAudioAggregateDevicePropertyComposition
        var setOutputDeviceIDsStatus = AudioObjectSetPropertyData(defaultOutputDeviceID, &property, 0, nil, outputDeviceIDsToSetSize, &outputDeviceIDsToSet)    }
        }
    }
}

In preparation, struct,

public Struct DeviceID {
    let deviceID: AudioObjectID
    let UID: String
}

and a Dictionary<String, DeviceID> consisting of them.

and code is

func makeAggregate (devices devs: Array<DeviceID>) -> AudioDeviceID {
    let deviceList = devs.map {
        [
            kAudioSubDeviceUIDKey: $0.UID,
            kAudioSubDeviceDriftCompensationKey : $0.UID == "com.rogueamoeba.Loopback:32B21BC2-536C-43A0-8A7B-8354B85AD4C7" ? 1 : 0
        ]
    }
    
    let description: Dictionary<String, Any> = [
        kAudioAggregateDeviceNameKey: "testDevice",
        kAudioAggregateDeviceUIDKey: UUID().uuidString,
        kAudioAggregateDeviceSubDeviceListKey: deviceList,
        kAudioAggregateDeviceMasterSubDeviceKey: devs.first?.UID as Any,
        kAudioAggregateDeviceClockDeviceKey: devs.first?.UID as Any,
        kAudioAggregateDeviceIsPrivateKey: 1,
        kAudioAggregateDeviceIsStackedKey: 1
    ]

    var aggregateDeviceID: AudioDeviceID = 0
    let status: OSStatus = AudioHardwareCreateAggregateDevice(description as CFDictionary, &aggregateDeviceID)
    print(status)

    return aggregateDeviceID
}

func applicationDidFinishLaunching(_ aNotification: Notification) {
    let devices: AudioDevices = AudioDevices()
    var audioFile: AVAudioFile
    print(devices.outputDevices)

    let deviceToBind: Array<DeviceID> = [devices.outputDevices["Loopback Audio"]!, devices.outputDevices["BoomAudio"]!]
    audioDeviceId = makeAggregate(devices: deviceToBind)
    print("new device \(audioDeviceId)")

    do {
        let fileURL: URL = Bundle.main.url(forResource: "1-03 RTRT", withExtension: "mp3")!
        audioFile = try AVAudioFile(forReading: fileURL)
        engine.attach(player)
        engine.connect(player, to: engine.mainMixerNode, format: nil)
        player.scheduleFile(audioFile, at: nil)
        try engine.outputNode.auAudioUnit.setDeviceID(audioDeviceId)
        try engine.start()
        player.play()
    } catch let error {
        print(error.localizedDescription)
    }
}

func applicationWillTerminate(_ aNotification: Notification) {
    let destroyStatus: OSStatus = AudioHardwareDestroyAggregateDevice(audioDeviceId)
    print("destroy done \(destroyStatus)")
}