Hi
We normally network buildings, and OR are very happy that's it's all ready.
As its only 2 points, one in each flat, I would myself, run cat5e to each flat, and cellar clearly mark cable to identify which flat it is, and in the flat, put a sunken back plate in, before the plasters do the walls, coil the cable and put a blank plate over the socket, hiding the cable.
That's job done ready for if/when needed
Many thanks
John