When you declare your many2many, you can choose/force yourself the tbl with the key relation.

More info here... http://odoo-new-api-guide-line.readthedocs.io/en/latest/fields.html#many2many

arel_ids = fields.Many2many(comodel_name='res.users',
                            relation='table_name',
                            column1='col_name',
                            column2='other_col_name')

Specific options:
    comodel_name: name of the opposite model
    relation: relational table name
    columns1: relational table left column name
    columns2: relational table right column name