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!
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.