Inversions

Wikipedia contains spoilers

Inversions, while not ostensibly a member of the Banks canon of Culture science fiction novels, deals explicitly with two parallel stories of outsiders in different cultures on the same medieval planet, the subtext being that these are two people from the Culture who have somehow ended up stranded on this primitive world.

(There are subtle hints in the text itself to this being a Culture novel, in the form of the capitalisation of "Culture" in the prologue and "Special Circumstances" in the epilogue - though these hints are made more ambiguous by having the narrator be fond of capitalisation in general - and in that it seems fairly clear that the Doctor has a knife-missile.)