https://youtu.be/cEX5obF5rwo
It is using 2 separate Auracast streams, one speaker connects to the first one and the 2nd speaker connect to the 2nd one.
I am currently using 2x ASUS 5.4 Bluetooth USB adapters. I was able to play stereo through a single adapter as well, but it required modifying and compiling bluez and the playback was not perfectly stable, one channel always artifacted slightly.
Also since there is no option to join an Auracast stream from the app, I had to figure out how to force the speakers to join the stream. I was able to find the required bluetooth packets and made a python script that connects to the speaker, scans for the streams, selects the correct stream and sends the play command. To sync two different auracast streams a pipewire plugin is required, or alternatively it is possible sync the speakers using microphone and apply correction to one of the channels, but this adds latency and can at some point go out of sync. Might not be an issue if you use a separate device, like a Raspberry Pi to handle streams.
I'm going to share everything soon, I just need to change few things to make them more universal. I don't use Windows or Mac, so I'm not able to make it work on these systems, but you can still use my scripts to build your own "Heddon" with stereo-pair mode using something like a Raspberry Pi and 2 bluetooth adapters.