I think that is exactly the problem. If you bake your Normals with half the Lowpoly, the Vertex Normals on the complete welded handle would be different along the seam to the mesh you baked with, resulting in a seam. To fix it either use different SGs on the complete Mesh, bake with the whole Lowpoly or fix the vertex…
I have usually have this occur when I bake from lowpoly to lowpoly in xNormal - try upping your angle bias (is that what it's called?) in your AO settings maybe.