I’m not sure that’ll work quite right. The seams would never come together well I don’t think. In addition to that you’ve 2 or more terrains being rendered then.
I suppose the best method still is to use ZBrush and sculpt out the cave as just a mesh. Kind of really sucks to still have to do this with latest and greatest engines. Would be great if Epic could chime in on this… maybe something already on the roadmap?
I think the solution is voxel boxes. They’d basically be zones that’d have the terrain behave like voxels so you can carve out holes, have overhangs, etc… they could also be completely independent from the terrain it self and just be a new type of voxel terrain that can be used along with traditional terrain for the caves, overhangs, etc…