A few months ago, I published a short story on KDP called The Double. It takes place in a world where every person is born with an exact twin. In this world, every person shares their life force with their double. When that double dies, their half of your shared life-force is reunited in you - the surviving twin. In this world, it is permissible to find your twin and kill him or her.
Today I came across this article on wikipedia. It is about the twins Jennifer and June Gibbons.
They were both ostracised from society and spent most of their adult lives living in mental institutions. At some point, they agreed that one of them would have to die for the other to live a full life.
Their whole story, especially Jennifer's death of heart failure, is shrouded in mystery and worth a look.