Absolute Terrain Combiner is a Unity3D Editor asset that helps you create and customize multiple terrain grid while keeping control on performances and optimization.. 
This tool helps creating a Heightmap and Terrain grid and blend elements only from a Heightmap texture and a color texture as input. 

Step1: setup the terrain grid (rows, columns, heightmap resolution), input an initial heightmap (PNG or 16bits RAW) and the tool will first help you to create a grid of heightmap at the adequate size. 

Step2: setup the terrain full size (Length, Width, Height) and the tool will then build the terrain grid from your height map, and will apply your color texture, eventually your normal map texture and will use the material you wish. 

Step3 (not included): optimize your scene as you wish with your own scripts to display the part of the terrain you wish and keep control on performances of your scene. 

A “Mars Gale Crater” scene is included with a 16bits RAW heightmap, a PNG heightmap, a color texture and normal map texture to demonstrate the process.

It it available now on Unity Asset store:

https://assetstore.unity.com/packages/tools/terrain/absolute-terrain-combiner-174578