So when you went into the ST app, you were able to see the ST and the Aqara Thread networks in “Manage Thread network” and the option to “Join this hub to another…” was available under the Unify Thread Network?
That’s the strange thing. I have four different Thread networks and ST only shows the Thread network for the hub I’m looking at. Do you know if you Aqara and ST hubs are using the same Thread channel? I know my Eero and ST are on different channels, but I don’t know if that would affect or not.
Now that you have ST, Aqara and Apple on the same Thread TBR mesh what do you see on pairing a Thread device? Do you still get asked which TBR to use? Is still necessary to add from one system to another with Matter (as with Matter over Wifi)?
Platforms do not care how many TBR are or which vendors make them, it’s just the Thread network name and the credentials. If the network has only the SmartThings hub or two additional TBRs from other vendors doesn’t change anything.
Yes, that doesn’t change anything either, Thread just provides connectivity to your local network, like WiFi, but is independent of Matter.
To use Matter multi-admin you have to share the device from a smart home platform like always.
The only difference of having one or more TBRs in the same network is redundancy and reliability. For instance, if the only TBR in your Thread network is the SmartThings hub but share a device to Apple Home, if the SmartThings hub dies, Apple Home won’t be able to control the device anymore. However, if you have more TBRs in the same Thread network, even if the SmartThings hub dies, Apple Home will be able to communicate with the device using the other TBRs.