However, just didn't fully understand the rationale to earth within one installation only. It would be almost impossible to insulate between the two earthing systems in this instance. 

It's just about protecting individual conductors. In our 2-way lighting example, you could take the cable into a Class II switch in the 2nd building and just terminate the c.p.c. to thin air (along with suitable precautions to prevent the switch being replaced by a Class I accessory). It's an option often chosen for things other than mains too - e.g. screens on signalling, data  or audio cables (Audio types often like to earth one end only anyway even within an installation to reduce mains hum) - the alternative would be a by-pass protective conductor in parallel to the screen(s) to shunt the bulk of the current past the screens. (Or use optical cables or wireless to design out the problem!)

Would you still suggest MET's were interconnected (LV side) given the transformer earthing mats are interlinked with bare buried conductor? 

Depends how well you can segregate what's supplied b the two different supplies - if you effectively have one installation spanning the entire building then 411.3.1.2 would demand a single common MET. If on the other hand you could keep exposed-conductive-parts of each side out of reach of each other (even if you had bonded extraneous-conductive-parts in common) you could treat it as two separate installations within the same building (like a terrace of houses). There again the steel frame might do the job of the MET for you (if sufficiently well interconnected) - making any extra bits of copper all a bit academic.

   - Andy.

However, just didn't fully understand the rationale to earth within one installation only. It would be almost impossible to insulate between the two earthing systems in this instance. 

It's just about protecting individual conductors. In our 2-way lighting example, you could take the cable into a Class II switch in the 2nd building and just terminate the c.p.c. to thin air (along with suitable precautions to prevent the switch being replaced by a Class I accessory). It's an option often chosen for things other than mains too - e.g. screens on signalling, data  or audio cables (Audio types often like to earth one end only anyway even within an installation to reduce mains hum) - the alternative would be a by-pass protective conductor in parallel to the screen(s) to shunt the bulk of the current past the screens. (Or use optical cables or wireless to design out the problem!)

Would you still suggest MET's were interconnected (LV side) given the transformer earthing mats are interlinked with bare buried conductor? 

Depends how well you can segregate what's supplied b the two different supplies - if you effectively have one installation spanning the entire building then 411.3.1.2 would demand a single common MET. If on the other hand you could keep exposed-conductive-parts of each side out of reach of each other (even if you had bonded extraneous-conductive-parts in common) you could treat it as two separate installations within the same building (like a terrace of houses). There again the steel frame might do the job of the MET for you (if sufficiently well interconnected) - making any extra bits of copper all a bit academic.

   - Andy.