I personally agree with this, and I'm sure many others that have learned and performed these pieces will. Some of the pairs are simply too contrasting to be played together, despite their shared key signature.
Besides the key signature, they are not. Tovey suggests in one of his commentaries that one pair in the first book is related (I don't remember exactly how; I believe he was only emphasizing the importance of playing both pieces together). Glenn Gould also famously stated that "a lot of the fugues in the Well-Tempered Clavier are better off without their preludes, and vice versa."I personally agree with this, and I'm sure many others that have learned and performed these pieces will. Some of the pairs are simply too contrasting to be played together, despite their shared key signature.
Given that in 1739 Bach published the exceedingly well organized and multi-layered Clavier-Ubung III, and then in 1741 the also exceedingly well organized and multi-layered Clavier-Ubung variations (the Goldberg, as we all know them now), it seems entirely implausible to me that when in 1742 Bach finished compiling WTC Book II and revising the then-20-year-old WTC Book I, that he would do so so that "besides the key signature, [the preludes and fugues of the well-tempered clavier] are not [related to each other]."Quaerendo invenietis.