There is a Telugu movie titled Savyasachi, which shows something called Vanishing Twin Syndrome.
How real is the depiction of the disease? Is it possible to have the consciousness of a twin in another twin's body, reacting through any body part?
This particular film premise is fortunately in the realm of fantasy.
When googling one can find stories of a 'vanished' twin apparently coming back, very early within the pregnancy of various levels of survivability, (I found only cases of living but brain dead fetus')
However there are no documented cases of a vanished twin that wasn't fully absorbed in their twin or turned into fetus papyraceus, or some state in between. But rest assured, not alive enough to have thoughts and the ability to control body parts.
The premise is often confused with Split Brain Syndrome.
After the right and left brain are separated, each hemisphere will have its own separate perception, concepts, and impulses to act. Having two "brains" in one body can create some interesting dilemmas. When one split-brain patient dressed himself, he sometimes pulled his pants up with one hand (that side of his brain wanted to get dressed) and down with the other (this side did not). He also reported to have grabbed his wife with his left hand and shaken her violently, at which point his right hand came to her aid and grabbed the aggressive left hand. However, such conflicts are very rare. If a conflict arises, one hemisphere usually overrides the other.1
As a TV trope, however I would say this an Organ Autonomy trope.
This trope is for when parts of the body have "minds of their own" and will control or influence the brain of whomever they're attached to.