Just to point out, one-episode production blocks have historically only been seen with double banking, associated with Doctor-lite episodes. The most recent Doctor-lite episode was Flatline. So my (very educated) guess is that we're seeing some form of double banking here, too.
Face the Raven is Clara-heavy, while Heaven Sent is hugely Clara-lite. Ashildr is only required for one episode in each of the two sets of production blocks. Though none of those episodes are truly Doctor-lite, are they? In any case, this could be a case of double banking. Not all scenes include Peter Capaldi as the Doctor, after all, so there could have been some clever timing involved here.
I've never before heard of a two-parter taking part in double banking of any sort, but if that's now a thing which has happened, we should allow for that case to be included in our definition by accounting for two 1-episode production blocks produced consecutively with another set of 1-episode production blocks, which is taken care of by a different director.
I'd really like some hard confirmation that this is double banking, though. Filming dates, perhaps?