>>410664026
In my experience merging meshes and then vertices at the seam, and then separating the meshes makes the normal good enough. It can fuck up blendshapes though. In Blender I'm basically a monkey pushing buttons semi-randomly until stuff seems to work how I want so take that with a grain of salt.

By the way, how are you checking for the seam? There was a configuration of viewport display that shows seams even if there wouldn't actually be one, maybe try playing with it.