What I think you should do is to select both the head and the body mesh and apply a skin modifier to both the meshes and then with the modifier being instance to each other, what you will do to one will be the same as the other. This is true until you bake the vertex of the skin modifier. Since both objects have different vertices count, it will not work and you might want to remove instance once you are done and continue weighting your vertex. You will run into times where the seams of the head and body doesn't match, there you will have to copy the vertex weight information from one mesh and apply it to another. With all that being said, why don't you apply a different material id to the head and the body and have it as one mesh? That way the head is using one map and the body is using another map and you don't have to detach them. It doesn't matter how well you do the vertex, you will always see the seams. Oh, and you can skin a character with the whole mesh, then apply a skinwrap to the head and body and use the character with the whole mesh as the driver. But then again, why go through all the trouble.