If you want to show data of two entities in a datagrid, you can not have a many to many association. The datagrid is always showing data for one entity and can show data of related entities, if there can only be a single object of this other entity. The relation needs to be 1 to 1 or many to 1.
If you really need to use many to many associations, it might help to have a helper association that is many to one, that points at the other object you want to show in your grid. But in order to do this, you need to add some extra logic to always set this association.