ALSA: How to duplicate audio so it comes out of two devices at once

Question

I have two devices playback_bt (Connects to BT chip) and aduplex_rtp (which delivers audio to a device/speaker over rtp). I can use aplay to play sound to them individually. I want to be able to write sound to the Loudspeaker device and it come out of both devices simultaneously. Below is my asound.conf. This must me done with just ALSA and preferably without using the multi plugin. This is on an embedded linux.

Here's a diagram. I want to figure out what I need to do to make the red lines work. [1]: https://i.sstatic.net/bmD9YhnU.png

My asound.conf

defaults.pcm.rate_converter "samplerate"
defaults.pcm.dmix.rate 8000

pcm.capture {
    type dsnoop
    ipc_key 1765
    ipc_perm 0666
    slave.pcm "hw:Loopback,0"
    slave.channels 1
    slave.period_time 20000
    slave.periods 2
    slave.rate 8000
}

pcm.playback {
    type dmix
    ipc_key 1766
    ipc_perm 0666
    slave.pcm "hw:Loopback,0"
    slave.channels 1
    slave.period_time 20000
    slave.periods 2
    slave.rate 8000
}

pcm.aduplex {
    type asym
    playback.pcm "dmix" 
    capture.pcm "dsnoop"
}

pcm.capture_rtp {
    type dsnoop
    ipc_key 1131
    ipc_perm 0666
    slave.pcm "hw:Loopback,1"
    slave.channels 1
    slave.period_time 20000
    slave.periods 2
    slave.rate 8000
}

pcm.playback_rtp {
    type dmix
    ipc_key 1132
    ipc_perm 0666
    slave.pcm "hw:Loopback,1"
    slave.channels 1
    slave.period_time 20000
    slave.periods 2
    slave.rate 8000
}

pcm.aduplex {
    type asym
    capture.pcm "capture"
    playback.pcm "playback"
}

pcm.LoudspeakerPCM {
    type plug
    slave.pcm playback
    hint {
        show on
        description "Loudspeaker PCM"
    }
}

pcm.NotificationPCM {
    type plug
    slave.pcm playback
    hint {
        show on
        description "Notification PCM"
    }
}

pcm.Loudspeaker {
    type softvol
    slave.pcm LoudspeakerPCM
    control {
        name 'Loudspeaker Master Softvol Control'
        card 0
    }
}

pcm.Notification {
    type softvol
    slave.pcm NotificationPCM
    control {
        name 'Notification Master Softvol Control'
        card 0
    }
}

pcm.aduplex_rtp {
    type asym
    capture.pcm "capture_rtp"
    playback.pcm "playback_rtp"
    hint {
        description "Duplex RTP"
    }
}

pcm.aduplex_rtp_plug {
    type plug
    slave.pcm aduplex_rtp
    hint {
        description "RTP Capture/Playback with Volume Control"
    }
}

pcm.capture_rtp_vol {
    type softvol
    slave.pcm capture_rtp
    control {
        name 'RTP Capture Softvol Control'
        card 0
    }
}

pcm.Microphone {
    type plug
    slave.pcm "dsnoop"
    hint {
        show on
        description "Microphone PCM"
    }
}

pcm.MicrophoneSoftVol {
    type softvol
    slave.pcm Microphone
    control {
        name 'Microphone Master Softvol Control'
        card 0
    }
}

pcm.!default {
    type plug
    slave.pcm "aduplex"
    hint {
        description "Combined capture/inject"
    }
}

pcm.bt-sco {
    type hw
    card BluetoothPCM
    device 0
    channels 1
    format S16_LE
}

ctl.bt-sco {
    type hw
    card BluetoothPCM
    device 0
}

pcm.playback_bt {
    type plug
    slave.pcm "bt-sco"
    route_policy default
    hint {
        description "Bluetooth Playback"
    }
}

pcm.mic_bt {
    type plug
    slave.pcm capture_bt
    hint {
        description "Bluetooth Microphone"
    }
}

pcm.bt_mic_vol {
    type softvol
    slave.pcm mic_bt
    control {
        name 'Bluetooth Microphone Master Softvol Control'
        card BluetoothPCM
    }
}

pcm.playback_bt_vol {
    type softvol
    slave.pcm pcm.playback_bt
    control {
        name 'Bluetooth Playback Master Softvol Control'
        card BluetoothPCM
    }
}

pcm.capture_bt {
    type dsnoop
    ipc_key 103
    ipc_perm 0666
    slave { 
        pcm bt-sco
    }
} ```