The How to: Implement Many-to-Many Relationships article provides a solution for a many-to-many association with XPCollection properties. If List<T> properties compose an association, a different approach is to be used. This is the case in Silverlight applications.
To create a many-to-many association with List<T> properties, use the ManyToManyAlias attribute. It requires an explicit declaration of an intermediate persistent class, and two one-to-many associations.