1. Install RaspiOS Lite
2. Install Docker or similarghcr.io/koenkk/zigbee2mqtt
3. create persistent /srv storage dir
4. use docker-compose (see below) to install these two containers
   • ghcr.io/koenkk/zigbee2mqtt
   • denniswitt/homeassistant-otbr:latest
5. configure Zigbee2MQTT using it's onboard screen.
   • Enable HomeAssistant AutoDiscovery.
   • Make sure to use a differnt mqtt-prefix than the other zigbee2mqtt instances
6. make sure you can see the OTBR multicast DNS broadcasts
   homeassistant should autodiscover it in the Settings/Threads panel
7. regardless, it's necessary to add the 2nd OTBR's API endpoint to homeassistant. In the menu of that panel, choose Add an OpenThreadBorderRouter. Enter the IP and Port of the 2nd Raspi
8. Finally, on the menu of the new router, choose “Add to main thread network”
9. Refresh webpage if it does not update automatically
10. It should look like this:

services:
    zigbee2mqtt:
        container_name: zigbee2mqtt
        image: ghcr.io/koenkk/zigbee2mqtt
        restart: unless-stopped
        volumes:
            - /srv/zigbee2mqtt/data/:/app/data
            - /run/udev:/run/udev:ro
        ports:
            # Frontend port
            - 8485:8080
        environment:
            - TZ=Europe/Berlin
        devices:
            - /dev/serial/by-id/usb-SONOFF_<Dongel you with to use for ZigBee>-port0:/dev/ttyUSBzb
    otbr:
        image: denniswitt/homeassistant-otbr:latest
        container_name: openthread-border-router
        restart: unless-stopped
        privileged: true
        network_mode: host
        environment:
            - DEVICE=/dev/ttyUSBth
            - BACKBONE_IF=eth0
            - BAUDRATE=460800
            - FLOW_CONTROL=0
            - BETA=0
            - AUTOFLASH_FIRMWARE=0
        devices:
          - /dev/serial/by-id/usb-SONOFF_<Dongle you with to use for Thread>-port0:/dev/ttyUSBth
        cap_add:
            - NET_ADMIN
            - NET_RAW