It may prove to be a fun exercise to hone your writing skills. You never know what your partner will throw at you, but it also helps if you have similar writing styles and an interest in the same genre you are working with.
All you have to do is write a paragraph or so and then your partner writes another and so on. See where your imaginations paired together take you. And don't worry if you don't like where the story is going, you can always throw in a random evil twin or something and take the story in a new direction. It'll broaden your horizons. :]