>>384627283
Very late but try joining the head and body meshes, then select a pair of vertices from neck and head and merge them. After that's done you copy the whole joined mesh and delete everything but the head from one copy, and everything but the main body from the other. You can also try data transfer modifier to transfer normals data for the neck vertex group between head and body.

If you want to check whether the seam is actually there in Blender I think you should enable solid shading in the upper right corner of your viewport, then set the lighting to matcap and colour to single, and disable outlines.

By the way, make sure your blendshapes and UVs are fine after you join and separate the meshes. Obviously makes copies whenever you try something like that.