I would suggest : >Keep your lowpoly intact and full, with middle verts properly welded >Unwrap as needed (UV splits shouldn't but who knows...), split the UVs down the center of the face, and move the unused part away to the right. (1 unit offset) >since the lowpoly is full, the vertex normals of the center line will…
Ah yeah there is your problem, what you want to do is bake the entire model, but on your lowpoly, just offset one side of the uvs 1 unit to the side. If you just have half the model there, your normals are all incorrect down the seam.