Editing Overlapping Terrains

I’m working on a scene that uses two terrain objects: a large terrain used for the landscape (the normal way), and a smaller terrain object that floats on top of the larger terrain. It’s used for details, e.g., crop furrows, too small to show in the larger terrain with good performance. The small, furrowed terrain is used for appearance and to place several GameObjects

The scheme works okay when they’re both flat, but now we’ve got to add matching rolling hills to both terrains.

I’ve come up with two schemes:

  1. Export the heightmaps of both terrains, and edit them in Photoshop, applying the same batch transformation to each heightmap.
  2. Apply the rolling hills to the larger terrain, and on start-up, distort the furrow terrain to match the rolling hills of the larger terrain using a script.

Neither of these seem like a good solution. Has anybody come up with a way of applying the same edits to different, layered terrains?

Another way (but very slow) is to sample the source HeightMap heights with GetHeights then feed that to SetHeights on the target terrain within your x,y bounds needed; it will take a long time (loop x, y) for that to work