I've done so many tests when trying to render enormous maps that include erosion.
My conclusion is that if you want a truly big map you have to forget erosion, in fact if you use tiled output erosion can produce enormous artifacts sometimes. The alternative is to render all the map in a normal build, but this requires computers so pricey and powerful that very few individuals can use them.
In an act of desperation I imagined a sort of blended mode, in which I rendered a reduced number of very big pieces of terrain that overlapped, hoping that I could eliminate the artifacts in the boundaries of the renderings by a careful planning of the overlapping zones. I expected the same output between two renderings in the part that overlap, but it's not like that: if you take two rendering areas that differ only a few meters then the regions that overlap will not look the same!
Yes, a major problem is that erosion is very context sensitive -- things that are quite far away can strongly influence local erosion. This makes sense -- after all, a piece of land that is unremarkable locally will be strongly affected if it happens to be in the drainage path of most of the map -- but is a troublesome element to square with tiled builds.
There's no perfect solution to this problem -- since any part of the map can strongly affect any other part, there's no way to get pixel-perfect results building only part of the map. Your idea of having overlapping zones is precisely what WM does to try to mitigate the tiled build problem. You can slide the blending slider upwards in the tiled build dialog to improve the artifacting situation -- this creates a larger overlap and thus smoother seams, but the results will not be identical to one large build.
Particularly erosion with geologic time turned on is susceptible to this problem.
Other solutions include:
1) Split your world into two files; this workflow is explicitly eased in WM 3. The first world does most of the heavy shaping and lifting, and is rendered as a single build. Your second world imports those monolithic files, does fine detailing (specifically, erosion without GTE on) and then exports as tiles.
This is easier in WM3dev because you can use a library file to hold all your outputs from the first world. Then create a new world file, use the "Synch Settings" file menu command to import the world settings from world file 1, and place a library input device to load in all the outputs in one fell swoop.
2) Turn off GTE
3) WM3dev includes an erosion reconstruction option that will keep erosion character from changing quite so remarkably at different scales; this helps with the tiled build is different problem, and might make simply turning your blend percentage up a totally viable solution.