Ideaspace: "Soulmates"
Two souls end up in the same body, and are forced to try to get along even though they have wildly different personalities and goals. "The Odd Couple," in a body!

(This was another story idea I came up with while walking to the store tonight. What sets it apart for me from similar stories in the past — including my story "Future History," which uses a similar idea — is the pun title: "Soulmates!")