hey bro, I'm not sure if this helps or if it's right

as far as i understand

One whole mesh has a total skin weight of 1 distributed from 0(black) to 1(white). so two seperate meshes would have a total weight of2. I don't think two meshes can share the 1 weighting.

If they are seperate and skined then you gotta paint wieghts seperatly. could have few problems.

If you unskin and connect the head to the body into one mesh.(vertex welding) then reskin. all should be better.

If you want to keep them seperate then cover the seam with something (like a collar)

hope this is sorta what you were afta

xian