I'm trying to configure OpenERP for a law firm. Let's say 2 customers (Bob and Jane) come to see a lawyer because they want him to write a contract. The lawyer will open a new file and will give it a number (for example, # 101-101). In OpenERP, that file will be a project called "101-101". When you create a project in OpenERP, you can link a customer to it. The only problem is that you can't link multiple customers to one project. In our example, I can link the project 101-101 to Bob or to Jane, but I cannot link it to both.
I faced the same question times ago, me too while implementing Odoo for a law firm.
Actually, if you focus on the result, the final target is to invoice the customer for the service - even if it sounds rude :)
From this point of view, you actually have TWO customers. Referencing two different projects.