I see. What I've typically done in the past with head blendshapes is use the character's clothes or some other obvious "divider" between the head and the body as a geometry seam. Like if the character has a shirt on, the shirt can cover the seam where the head geometry is separate from the rest of him. Even if that means the head geometry goes partway down his chest, the seam will be hidden by the covering clothes or whatnot.

If you're not planning on using clothes, you could try doing as I suggested (combine, merge, soften) and then extract the faces. Hopefully the normals will still match enough after the extraction.

Another option would be animated the face with a skeleton rather than blendshapes, but I can understand if that's not an ideal alternative.