Got your PM. The last post gave us a better idea of what you are trying to do, so here is one way...
Here is a link to my main discussion on tiled terrains.
http://forums.planetside.co.uk/index.php?topic=2036.0The main problem you're going to have is disguising the border between the two terrains since the elevations will be markedly different. Given that the border will be in the distance it should be relatively easy to disguise it by a) hiding the border behind some hills closer to the camera or b) creating a broad blending shader between the two.
I've attached a quick example using my terrain blending mask... the mask was ready made. It combines a TER file of Glacier National Park with a fractal terrain (displacement of 500). I've cropped the 8194 terrain by 50% with a broad blend. Rather than merge the terrains I just use the mask as a blending shader on both (inverting the shader for the fractal terrain) The end result, a fractal hill with DEM derived peaks in the distance.
You may need to make a more appropriate mask for what you are trying to do but the principle is the same.