Random jarthur shared body hcs that I have because I can't stop thinking about them:
Even though he can't see, and John is usually the one to move them around, Arthur technically has primary control over their eyes, and oftentimes, muscle memory (think like rolling his eyes when he's annoyed, etc., etc.) will make him move his eyes unintentionally. The involuntary movement part of it makes John really nauseous/motion sick but he doesn't know how to describe the feeling so he just calls it annoying up until Arthur gets really sick and describes the impending doom and full body illness of feeling like you're about to throw up and John's like "oh so like what happens to me when you move my eyes" and Arthur now feels really guilty about it cause he didn't know it way actually making John sick
Also on the eye vein, while John usually controls the eyes, the eyelids are fully Arthur's area and as he becomes more used to not really thinking about his eyes, he starts to forget that he needs to blink so their eyes are chronically dry. John tries to ignore it until their eyes start watering from how dry they are, and then he yells at Arthur that they need to blink. Sometimes the reminder also comes from other people who think it's weird that Arthur hasn't blinked in an absurdly long time
As much as cutting off part of his pinky in the Dreamlands hurt, John maintains that the worst pain he's ever experienced was the one time that Arthur accidentally hit his left elbow on the corner of his desk and John felt like his whole arm was on fire for like five whole minutes. He will never forgive Arthur for this incident. Every time their left elbow gets even slightly close to another object, he freaks out and tells Arthur to move it. His second-worst pain was that one time Arthur stubbed his toe on a table leg.
John is very sensitive to temperatures, and because of that, when it's cold out, he is very insistent that Arthur wear extra layers on the parts of their body that he feels (left arm & foot). The only hot thing he'll tolerate is hot water when they wash their hands, but even that he's not a big fan of. He will refuse to touch any object that has a temperature that is not equal to room temperature unless there is absolutely no way Arthur can safely handle it with one hand
Because he cannot feel his left foot, Arthur drags/catches that foot significantly more than the right, but he denies this happens until John makes him feel the heels of both shoes, and the left is significantly more worn down than the right
When they are sitting, if the task does not require two hands, Arthur will lean his head on his left hand and let John play with his hair because John likes the texture of it, and Arthur thinks it's soothing for other people to mess with his hair even if it's technically his hand doing it
Arthur writes with the right hand while John writes with the left, and they are fully capable of writing about completely different things simultaneously as long as they aren't talking about one thing or the other. Often, this manifests as Arthur writing notes on the case they're currently working on with one hand while John jots down ideas for his stories with the other, and it looks utterly insane from the outside. They also have vastly different handwriting styles (Athur's is very small, and his letters are pretty cramped together, John's is a lot bigger and has a lot more flair; John does, in fact, complain about having to read Arthur's handwriting on the regular). One time they left some of John's notes sitting out on the desk and Faroe asked about them because it is so obviously not Arthur's writing. He panicked and told her it was accidentally left by a client, so now they have to make sure John's stuff is hidden when she comes over to avoid that conversation again