Lets create a complex lightmap - Open discussion

Unreal needs a lightmap that is not overlapping, has no stitched hard edges, has good sized gaps and doesn’t stick out of the main uv square. If your regular/first uv channel does all that then you don’t need a 2nd one for the lightmap. You can set the channel in the mesh property window in the static mesh section. No need to do anything in the material.