If John And Arthur Ever Separated, I Don't Think They Would Ever Be Able To Live Without Each Other.

If John and Arthur ever separated, I don't think they would ever be able to live without each other. The amount of codependency in their relationship is too much any human or now human to bear.

I see two possible outcomes, either way, one of them cracks and acts up on their desire to become whole again.

1: If John loses his mind first.

When he gets his new body and has already learned coordinating it, (it would take so much time and physical and emotional effort to do so, but that's another story) he would feel too alone even lost. His entire existence as we know now was with Arthur, at least in the human world. He was so afraid to go back to the dark world, he couldn't bear to be alone again. He couldn't hear Arthur's thoughts, nor could Arthur need his assistance anymore. He would be completely on his own. This is enough in itself to go mental, but what if he would feel so alone that he would see the only option to became happy as going back. But there is no going back. He would find a way: cutting open his soulmate's body and getting into him. Once again they'll be whole, and he would be inside him as he should.

2: Arthur looses is first.

When John gets a body, Arthur has to learn to be on his own again, either blind or with eyesight he would lose his guide. Even if he can see again, the world is not the same as John described it, it's different and less colorful.  If he can see again, he will see how much damage their adventure done to his body. He is half the man he was before, and no one ever in the whole world could see him and know what he feels other than John. His emotional support is gone, but that was the only thing that got his self-esteem going. He tried, or was on the edge of killing himself before, but now he is too close. He can't even bear to see John, with his new body; no scars, no damage. Not only that, but he had to go through so much, and only he stayed the same? He needs John back, but with a body he needs to put it back someway.

He eats him. Every little piece of him. The hair, the bones. Nothing can leave from him, so they can go back to normal, no John's body, he is John's body, and he is inside of him again.