There is surely need for common sense. I think that all off-the-shelf double sockets are configured either with one socket technically 'a spur' from the other or, if the current path splits, as two separate single sockets. In either case, a pedantic view would presumably say that they could not be used on an unfused spur - which would make nonsense of the regs (well, an informative Appendix thereto), which indicate that a double socket on an unfused spur is acceptable!So when creating a spur from a spur, can you tell me where it is specified the maximum distance of wiring that may be allowed beween socket 1 and socket 2. Are you saying that 5cm is OK, but 10cm is not? ... In my book, and according to the regs, a spur cannot have another socket connected to it.
I can but presume that the thinking behind Appendix 15 is that two 13A outlets on the same faceplate are 'unlikley' to be loaded to more than 20A (the minimum CCC allowed for ring final/spur cable) for 'long periods', whereas that may be less true of two 'separate' 13A sockets (even if in very close proximity).
Kind Regards, John