Permaculture in New Zealand

Imagine the following situation: A couple owns 100 acres of land; they have built a house there and established an organic vinyard. they would now like to share this land legally with at least one other family. They don't want to subdivide, but they want to know that if something happens to them, they can move on (i.e. sell their share and their house &business for a fair price) whereas the other family can stay happily ever after if they wish.
Have you got real life experience with a legal structure that would allow for security on both sides? Or do you know a lawyer who has experience dealing with such situations?
The land in question is within the Tasman District Council, and the people involved are keen.
Your help is needed urgently! ;) thanks