PieterPan- Only one RA signals the throttles to close in Retard. I'm not sure which. However, failure modes of indicators are designed not to fail at critical readings. RA is a digital system with self confidence tests.
Thanks for the clarification, I've heard that it is the captain's. Still strange that the system doesn't cross reference the other altimeter. In this case all the holes in the cheese lined up. I will not further speculate as I don't know the systems involved.