In all honesty, I’ve never needed to blend them. One generally big terrain has worked fine with my with proper calculations of World Machine nodes. I export in the highest possible settings and later on scale down to decided size. Importing the Raw32 to UE4 and determining tri-count from there is straightforward but it depends on how big your terrain is. Remember that most modern GPUs and drawcall millions of polygons, so the World Machine output will generally be fine.
I also recommend checking out this documentation for the height/slope outputs in order to properly import to Unreal.