As I see it, failure of a NRV or the fuel transfer pipe would trap fuel in the main tank, starving the supply tanks and ultimately the engines.
The FAA fuel system requirements are:
Fuel System Independence
Each fuel system for a multiengine airplane must be arranged so that, in at least one system configuration, the failure of any one component (other than a fuel tank) does not result in the loss of power of more than one engine or require immediate action by the pilot to prevent the loss of power of more than one engine.
Eurocopter being such a huge company would have this covered, ....wouldn't they?