Bodysharing

Bodysharing is a plot device or trope where two (or more) people's minds share the same body. 
Usually in a bodysharing scenario the different minds in the body alternate who is in control of the body, unlike in bodypossession scenarios (as in Goa'uld possession in Stargate, or demon possession and angel vessels in Supernatural, etc.) where one of the minds is subsumed to the other and no longer gets to control the body.
==Canon Examples==
* D.N.Angel is a manga based on the premise that the hero, Daisuke, shares his body and turns intermittently into the thief Dark Mousy. Despite the difficulty of conducting a romance in this fashion, there are a few Daisuke/Dark Mousy stories.
* In Harry Potter, Voldemort shares a body with Quirrell, possesses Ginny Weasley and leaves soul fragments in Harry and a giant snake
* In Naruto the titular character unwillingly shares his body with a nine-tailed demon fox, the Kyuubi. A number of fanworks turn around Naruto coming to terms with this. 
* The third Star Trek (TOS) movie, Star Trek III: The Search for Spock featured Spock's ghostly mind hanging out in Dr. McCoy's brain. Body sharing also happened in episodes of Star Trek (TOS) and other parts of the franchise. 
* In Star Trek: Deep Space Nine, the Dax symbiont inhabits joined Trills Jadzia and Ezri, sharing personality and memories.
* In Stargate SG-1, the Tok'ra (the Goa'uld's more benevolent cousins), share bodies with their hosts, who are volunteers, rather than forcibly possessing them and allowing them no control the way the Goa'uld do.
* The Stargate Atlantis episode Duet featured Rodney McKay and Laura Cadman sharing Rodney's body upon faulty retrieval from a Wraith dart ship. The expedition's mental health counselor talked to them about how to share it. (This episode led to a lot of fic.A small sample of Duet-related fics listed at LiveJournal community SGA Episodefic.)
* Yu-Gi-Oh! tells the story of a boy who is possessed by another personality; eventually they come to work together.
* In Dragon Age II, Anders is a mage who acts as a willing host to the incorporeal spirit Justice, altering his personality and concerns.
* In Elden Ring, Queen Marika is shown as sharing a single body with her male counterpart/alter ego Radagon, though it is implied they were physically separate at some point in the past. Melina informs the player that Marika once told Radagon: "Thou'rt yet to become me. Thou'rt yet to become a god." The game provides an explicit canonical example of soul-sharing with the characters known as D, twin brothers described as having "two bodies and two minds, but one single soul."
==Fanwork Examples==
* Harry Potter - The Semblance of Peace by Maeglinyedi - Harry & Voldemort body-share. One of the best known of the many Potterverse stories on this premise
* Sherlock - It Feels Like Home When I'm With You (link) by etothepii - Sherlock likes ghost!John enough to try giving him control over his body for a bit. 
* Stargate SG-1 - The Standard of Comparison by agentotter - Jack and Daniel share a body. It doesn't work too well.
* Star Trek: Deep Space Nine - Symbiosis by nostalgia -- Dax & Ezri and the bizarreness that is a joined Trill
*Star Trek: the Next Generation - Hope of Thee (the Zombie Remix) by Beatrice Otter - Lwaxana Troi shares her body with her dead daughter.
*Supernatural - Live In Skin verse by flawedamythyst. To keep Dean's soul from going to Hell, Sam decides to bind it to his own body, and that means sharing. (gen fic)
*Assassin's Creed - Thirty-Three by devovitquesuasartes/devovitsuasartes - Clay is saved from deletion by hitching a ride in Desmond's body.
* Scum Villain's Self-Saving System - It's reasonably common to find fics involving the transmigrator main character Shen Yuan sharing the body of the character he transmigrates into, Shen Jiu, instead of fully taking it over (which could be viewed as 'killing' Shen Jiu in the process).
* Stories tagged Sharing a Body at the AO3
==References==