How do I blend terrain prefabs together?

So I have a terrain prefab, and I want drag it into the scene right beside the terrain the prefab was made from and have them mesh together.

I've tried using the paint raise/lower height terrain tools, and unpacking but with this line of troubleshooting the painting I do to one instance tile applies inexplicably to the prefab tile as pictured below. How do I blend terrain prefabs together?