Indeed - but, as I said, exactly the same regardless of how the heating CU is fed.With this setup all of the heating can be isolated simply and easily to the uninitiated - simple to understand.
What 'third unit' are you talking about - the Henley block, perhaps, or are you thinking about having a 'whole house isolator' as well as the main switches in each CU?The system also has a main switch as well, so all is off in the house. Could have both CUs off Henley blocks and a separate mainswitch for both, which produces messiness with three units on the wall.
Kind Regards, John