when I started with terrains (before i learned to splice them) i saw that no matter how far away a blue(or whatever sky texture you use) line (or even a tiny dot) in the middle of terrains is very noticable.

I think your idea of the border vertex z value halved to hide the seams is the best option. even if they are not sewn together it will look like it, and will be much less painfull than using those "mid terrains" i talked about to sew them perfectly. I would go with that, because your concept is much simpler to program and probably achieves a good result also