bemade-addons/models/sale_order.py

from odoo import models, fields, api, _, Command


class SaleOrder(models.Model):
    _inherit = 'sale.order'

    billing_contacts = fields.Many2many(comodel_name='res.partner',
                                        string='Billing Contacts',
                                        compute='_compute_billing_contacts',
                                        inverse='_inverse_billing_contacts',
                                        store=True)

    @api.depends('partner_id')
    def _compute_billing_contacts(self):
        for rec in self:
            rec.billing_contacts = rec.partner_id.billing_contacts

    def _inverse_billing_contacts(self):
        pass