Overlapping terrain with different resalutions

I have a large (huge) terrain for a fly over that will transfer into a close up which will be a seperate terrain with higher resolution. These terrains will esentially be the same shape causeing overlap. My question is what is the best way to merge these terrains as seamless as possible. I was thinking creating a black square (or shape of terrain that is being merged) in the hightmap in the large terrain creating a “hole” in the large terrain for my smaller terrain to fit in? i havent tried it yet but am thinking i will have problems with reselution of the big map not “clipping” the hole propperly

ended up cutting out “holes” in photoshop. this works well but leaves a seam, but is not impossible to hide with texture blending/ veg ,grass cover up.