The BT Business 'smart' Hub 2 appears to be one of the least technically-documented pieces of IT equipment, with BT apparently trying to avoid anyone messing with its configuration.
indeed,When i bumped into one
I could not believe the features it had.. and completely undocumented.
and IPv6 support
BT hiding their light under a bushel...
including that (on the Business hub) you can define your Custom Branded Guest wifi, not simply take the BT Wifi option. or just turn it off completely (something not possible on the Home version without a call to customer services to disable it remotely), else it can suck all your bandwidth unknowingly.
That said, for 2 companies, needing complete isolation,
Simpler would surely be 2 contracts, 2 routers. etc.
Or as you suggest a Draytek or similar that can NAT into two separate subnets and set it to NOT route between them.