SOTO wrote: And if the "common setting" amendment seems like stretching the simple rule, it's no loss to consider Heaven Sent and Hell Bent separate.
It seems more like a The Girl Who Died/The Woman Who Lived case, where there's a linking element and similar titles, but totally separate plots. Anyway, we shouldn't squabble over individual stories. We want rules that are clear, and then we want to stick to those.
Under your list of qualifications, TGWD/TWWL would be an unambiguous two-parter despite the separate plots... exactly what we intended to prevent with AGMGTW.
With that being said, TGWD/TWWL has more of a case as a two-parter than either AGMGTW link does.