Mesh Overlap Ugliness - Please help


I have a problem with two mesh objects being overlapped to build a landscape from separate components.

Where the objects overlap, there is a rough looking seam. The seam is only really noticeable in the light. In the shadows it can’t be seen. The weird thing is that if I “merge actors” this ugly looking seam isn’t there. Does anyone know how to avoid the rough join, without merging actors?