Many2many should be always bidirectional.

When you create a many2many field the other object linked should automatically receive an entry to this many2many field too.

To avoid such a strange thing like in http://openobject.com/forum/topic11596.html#37730

Create an object, the other with the link to the first one, then inherit the first one to make the link with the second one.

EDIT: Another alternative is to use "backref='myBackRef' in arguments"
like in SQL Alchemy: http://www.sqlalchemy.org/docs/05/ormtutorial.html#building-a-many-to-many-relation