Hello,
The difficult taks here is:
for both objects find the UV location of the intesecting 3D surface and then average (to make them equal → smooth ) the normals of both UV maps.
Also even for the “nearest” locations with a lesser facor of smoothing.
I’m not aware of any general usable technique or algoritm to do so.
Idea: with geometry nodes one might use the Geometry Proximity node to find the “influencing” ares… but i have yet not any idea about the normals…
Edit: a little bit of thinking and trying later…
Hmm… in fact backing the “color” from one object to another using the normal values and then mixing this with the proximity value (not the red/green) and the origianl normals (not yet) might do this…
Duplicate the whole uncut model. If its already cut into pieces - join it and weld. Make sure it smooth and no seams on shading are appear, model scales, normals and not inverted, etc…
It will became a “target”:
Now we have a “target mesh” + 2,3,etc., different pieces which will show some seams because of differences in normals:
Make a vertex group for all pieces mesh borders like so, and use it in data transfer modifier:
Result:
Also: for me the colors in your screenshot are looks like if normals for lower part are inverted, perhaps
But as i understood this the “problem” was not between the body and the neck but between the body and the breasts. Which seems separate objects with not the same “border” on both sides but more like overlapping objects like in the comceptual blockout phase (before joining and blending by sculpting).
Also when the borders line up then this does not mean that the normals will when “simply” extruding to somewhere" (so to speak)… so some additional “mixing” might be needed depending on the actual result or need of precision.
thanks for the replies… I’ve finally found a good tutorial on this and achieved a somewhat decent result. Here is the link if youre interested
I never noticed this modifer… 
Thanks… i learnt something new today 
