Does anyone know if this is possible using AVFoundation?

I have created a slider that generates a tone. I need it to generate two tones but xcode is producing a plethora of errors any way I attempt it - is this impossible?

Here is some of the set up I'm using:

    tone = AVTonePlayerUnit()
    slider.value = 0.0
    let format = AVAudioFormat(standardFormatWithSampleRate: tone.sampleRate, channels: 1)
    print(format.sampleRate)
    engine = AVAudioEngine()
    engine.attachNode(tone)
    let mixer = engine.mainMixerNode
    engine.connect(tone, to: mixer, format: format)
    do {
        try engine.start()
    } catch let error as NSError {
        print(error)
    }

EDIT:

I have solved the problem by creating a separate class for the AVTonePlyerUnit. I have a separate problem with creating a volume slider for the generator.