I used a Nest Hub as the first one (primarily b/c I was having a 
time trusting that I could tell IT what Thread creds to use, so I settled on letting it have its say and also using a spare phone that I could wipe the credentials to know exactly what’s being stored).
From there I shared the devices to Home Assistant where I could easily tell its TBR to use the same Thread creds and share the mesh. A single Thread mesh with 2 fabrics and 2 controllers.
Neither of those hubs support binding. HA intends to, when they come up for air. So not in the immediate future.
Instead I installed chip-tool on a Linux machine and shared the devices onto a third fabric and used chip-tool to handle the bindings. I wrote up the commands I used over here: Multiple Matter/Thread Dimmers should control one load - #6 by Mactalla - Matter/Thread - Home Assistant Community.
Happy to go into more detail if you wish, but probably in its own topic rather than this bug tracking thread 