In the Mistborn series, some people can heal themselves if they stored “health” in gold (usually in the form of jewelry) prior to getting injured. The way it works is you only heal back to whatever state your mental cognition agrees constitutes your body just prior to getting injured. So if you had lost an eye a decade ago and now wear an eyepatch, and accept that that not having a working eye is what makes you you, but then get stabbed in the gut and need to heal, you will only heal the stabbed gut, and not the eye.

And then also, what if you cut somebody down the middle and cast it on both halves (say at the same time)?

Reminds me of an idea that I'm stealing from an anime for a warforged character. If you can make prosthetic versions of every organ and limb, and the brain can survive being cut in half (as it does for some folks with epilepsy)... What if someone had a full half of their body replaced by prosthetic limbs and organs, and the half of their body that was removed also got prosthetic limbs and organs, and you put one half of their brain in each body? Do you have two of the same person, or are they two different people now? ...If one body dies, it's unreasonable to think the other would too. So does each body have its own soul? Did we artificially make a soul?

Now put the human back together. Did two souls merge, or is there still a soul floating around?

...now put the prosthetics together. Does it now house that hypothetical soul? (Spoiler: it does in my campaign)