True. Although 2 x 2.5 mm² T+E cables in parallel would obviously have a higher CCC, and a lower R2 for a given length, than would 4mm² T+E, I was not suggesting that one should consider it as any 'better' than 4 mm² T+E. As you say, if one did want to use it as 'more than 4 mm² (see *** below)', one would have the problem of its CCC not being available from the usual tabulations.yep, ... if you treat 2 x 2.5 as 5.0mm conductors and a 3.0 earthwire then you will not be far wrong, although a little bit of interpolation on the current carrying capacity, volt drop and expected Zs might be required because they are non standard sizes to look up in tables. The nearer the lengths match the better your "guess" would be.
[ *** interpolation between CCCs given for standard sizes of T+E suggest that the CCC of 2 x 2.5mm² cables would, if it were included in the tabulations) probably have a CCC (with Method C) of a bit over 40 A (maybe appreciably higher, because of different heat loss behaviour when there are two cables) - so a 40 A OPD may well be acceptable, but one would presumably have to find some 'official confirmation (or strong evidence of one's own) to confirm that ...
Kind Regards, John