Q: How difficult can it be, to have a second open thread border router #otbr docker container, running on a second pi, join the same thread network and connect #matter #iot devices to #homeassistant?
A: many MANY hours worth of trial and error and setting up different images until you suddenly don't know what the problem was.
1. Use the right container. Derived from the HA one. Not the official docker otbr
2. Make sure your multiple containers which manage other USB sticks, like Zigbee, don't interfere with your radios
3. Make sure mDNS is being announced AND yet also manually add the API Url
4. Add the new thread router appearing in your HA to your thread network.
5. Refresh HA/App webpage
realraum