Hi,
The syntax for Many2Many fields is :-

    fields.Many2many(comodel_name=None, relation=None, column1=None, column2=None, string=None)

Here comodel_name is the name of target model,
and relation is name of table that store relation in database. ie it will create a table in database with the given name, which store the relation,
column1 and column2  are the name of columns of relation-table in database.

If you are talking about to and fro relation between two objects(tables). Lets assume that our objects are teachers and subjects, and we want a to and fro relation, i.e.  we can add teachers to subject from subject-form-view and we can also add subjects to teacher from teacher-form-view, either way it have to reflect in both sides. This we can achieve using same relation-table for both Many2many fields. For more understanding, refer code

class Teacher(models.Model):
    _name = "teacher.teacher"

    name = fields.Char(string='Teacher Name')
    subject_ids = fields.Many2many('subject.subject', 'teacher_sub_rel', 'subject_id', 'teacher_id', string='Teachers')

class Subject(models.Model):
    _name = "subject.subject"

    name = fields.Char(string='Subject Name')
    teacher_ids = fields.Many2many('teacher.teacher', 'teacher_sub_rel', 'teacher_id', 'subject_id', string='Subject')

Hope this helps.