I think the main problem here is that the a deficiency in the usual terminology, which frustrates attempts at clarity in anything other than very simple situations ...
A simple ring or radial final circuit will have a number of physical conductors ('wires'), joined together at accessories etc., and it is common to describe the totality of those joined wires as "the CPC" of the circuit - but that leaves one needing a word/phrase (other than 'CPC', since that is now spoken for) to describe those individual component 'wires' - unless one just calls them 'wires', I can't think of anything other than "protective conductor" - can you?
If, as in the system you postulate, there are two 'rings', a problem of terminology arises. If one is normally using "the CPC" to represent the totality of the system of 'connections to earth' then there would still only be 'one CPC' when there were two rings. Indeed, if one uses "CPC" to have that meaning, then no circuit could ever have more than one "CPC", could it?
If one removes any possible quibbles by postulating a hypothetical socket with four earth terminals, then, if one had 'two rings', it would actually be impossible to say which 'wire' belonged to which ring - so it would really be a question of 'doubling up' conductors in parallel, rather than having two physically distinguishable discrete rings.
Of course, this is all tangential to the actual issue under discussion, but an inadequacy in available terminology clearly doesn't help at all if one wishes to work strictly to the word of what the regulations "actually say". In fact, in the absence of adequate, properly defined, terminology, all one can really hope to work to is what the regulations "appear to say" - and that obviously opens up the possibility of varying opinions.
Kind Regards, John
