We operate our body with a single processor, the brain. Our organs are also managed by the brain. The brain is highly optimized to operate each of our organs, control motor functions, and allow us to think.
1 brain cannot control 2 bodies, there’s simply too much going on. Likewise, 2 full brains cannot control the same body, they would fight for resources. Organs keep the brain alive, the brain keeps organs alive. It’s a 1:1 ratio, sometimes less than 1, but never double (some have been born with extra controllable appendages but it’s few and far between).
Therefore, only a higher, more generalized, processor could handle operating 2 or more distinct beings.
If only our cells could talk.
This makes me imagine a sci-fi scenario where it’s possible to add the ability to control additional bodies to a persons brain, probably using some kind of radio-like remote control network, but only when they are young enough to have a high enough brain plasticity. After a certain age, you lose the window of opportunity to add additional bodies to your brain.
Hmm, now I kinda want to read Ancilliary Justice again.