Hi alistair.fox,
Smart Lists are essentially saved searches, and contain no tasks themselves. This means that they return the results of a search based on the tasks in your account. When the topic of sharing Smart Lists arises, there are a few ways that this could be implemented. If you haven't already, you may want to take a look at this thread in our Ideas forum.

In the meantime, if you and your contact are already sharing some lists, and you want to create a Smart List based off those shared lists, it's possible for both of you to create your own Smart Lists on each account. If they are based on the shared lists, the searches will return the same results.

Hope that helps!