Positioning of the house on a north plot

What happens to the building line if I place the house on the boundary of my two plots? [...] Does it still count then?

We can’t even dream in our wildest school wisdom about what applies in THIS CASE. Your building line is obviously something special, almost a special creation of your building authority. Actually, it would count for the development of this one plot, and in case of a merger accordingly for the new combined plot of both. But apparently your building authority is no less willing to make exceptions with this building line than at most with a building boundary. That’s why I leave it to the gods to understand why a building line is provided here at all. If it is meant only as an expression of a wish, it could just as well have been ignored.