If you want to connect multiple midiboxes via USB you would need to define different USB descriptions in the firmware which means you need to be able to compile firmwares by yourself.
I use direct midi uart coupling between core at 10x speed, not optocoupler.
For this you need:
Same PSU for all core (my 3 core are in a 3U rack)
Short wire between core J11 connectors
Routing configuration at “master” core where midi i/o (to/from slave) go to USB midi 2 to 4
My protocol emulation is MCU
Now for your specific configuration, If cubase only allow 1 HUI device to be configured, I suspect all your controllers will be mirrored, whatever chaining/connection between controllers you use…
Are you sure you can’t use MCU protocol which usually allow 4 units (1master+3 extanders) to be configured in most DAW