Hey just off the top of my head. You could either export both height maps or meshes from WM into zBrush, put them on the same layer and zRemesh / Dynamesh them together. Export the now combined two meshes back into WM via a File input node. At this point you have both original maps combined into one terrain, but you'll have to work on the detail of the seam you connected in zBrush. I'm sorry if this is totally off, but good luck otherwise l.