The Yamato way is how I do mine or how I learned to do them and now it's quite easy when I want to do them.

I set up the two figures together in a scene, then adjust till I get the neck and head of the one I want on the body and neck of the other. I save that scene out to come back to later. I then export each neck out and import them into a modeler. I then cut the two in half or so and slice them together adding the needed verts to match. Then export the new neck back into the saved poser scene.

Import the new neck. It should fit in place. Now export the new neck and the body of the one you just want for the body and the and head of the other. Uncheck * as morph target" and check the rest. Save as a new Name.obj in whatever geometry file you want.  Save the figure that is still in the scene into your new figures runtime folder. This creates the new cr2 that your going to use.

Open the new Cr2 in word or notepad and change the path of the .obj to point to the new on. There are 2 lines that need this. Now you can clean the rest of the Cr2 to get rid of the unwanted groups of both figures that you don't need. All morphs will work except on the neck because you changed the vert count. Some JP adjustment on neck and head may be required.

Easier than it sounds to do.

The other way is to line the 2 figures up and turn the head of on invisable and the body of the other invisable and use magnets to blend the 2 necks. Save the figure and there you go. Adjust the JP's. You can do much of the same cleanup if you want.

You will need to post work where the two necks join unless you make a new uv map or adapt a texture to work with the changed neck.

Again very simple once you do it a couple of times.

My Miki/V4 has the new neck and textures adapted to work with it.