Just curious, is there a reason you don't want to combine the meshes together? If you combine the mesh and then merge the vertices at the seam (so that instead of having 2 stacked verts along the seam, you merge them down to 1), it should fix the issue after softening the normals on the new edge that is created.
The reason are blendshapes on the head. I might to extensive modifications on the torso in the future, and having head and torso separated saves me from a lot of potential headaches. Thanks for your advice!