Haunted by the other 'life'

Both versions of this story begin centre on the the theme that each possibility of our life is equally legitimate. There are endless other lives, universes, or threads. We might have become these, or we may one day become them.
And in this story all the characters are in one way haunted by these other lives. I am deliberately playing on the horror/thriller genre here.
A recurring incident in Sophie's and Paul's story: one night Paul spys a car parked below their hi-rise flat. The engine is running and the wipers wave back and forth. Though it is impossible to see clearly because of the dark and the rain, both Paul and Sophie believe that the driver is parked there looking up at them in their flat, and that the driver is a woman. Paul is angry and threatened. He wants to confront the woman, though he cannot say why. Sophie is frightened and would rather hide in the bedroom than confront her.
At the end of Paul's and Sophie's story Sophie is again at the window and the car is below.
In the car is Sophie too. Another thread. She is not with Paul (or with anyone). She lives in a house (not a flat) and struggles for direction (unlike the first Sophie who is so certain about what she is).