Python Code:

class ProductTemplateInherit(models.Model):
     _inherit = 'product.template'

x_attribute_line_ids = fields.One2many('product.template.attribute.line',
                                        'x_product_tmpl_id', 'X Product Attributes')

class ff_product_template_attribute_line(models.Model):
   _inherit = 'product.template.attribute.line'

   main_line_id = fields.Many2one(comodel_name='product.template.attribute.line')
   x_product_tmpl_id = fields.Many2one('product.template',
                                     string='X Product Template',
                                     ondelete='cascade', required=True)
   x_attribute_id = fields.Many2one('product.attribute', string='X Attribute', ondelete='restrict', required=True)
   x_value_ids = fields.Many2many('product.attribute.value', string='X Attribute Values')

​And On Create i add:

@api.model
def create(self, vals):
   """
   - On create a record related to product_tmpl_id add same vals on ff_product_tmpl_id
   :param vals:
   :return: SUPER
   """
   print("VALS:: ",vals)
   rec = super(x_product_template_attribute_line, self).create(vals)
   if vals.get('x_product_tmpl_id'):
      new_vals = {
         'product_tmpl_id': rec.x_product_tmpl_id.id,
         'attribute_id': rec.x_attribute_id.id,
         'value_ids': [(0, 0, rec.x_value_ids.ids)]
      }
      print("New Vals: ", new_vals)
      main_line_id = self.create(new_vals)
      print("main_line_id", main_line_id)
      rec.main_line_id = main_line_id.id
   return rec

but it doesn't do the needed result, and keep showing errors. 

on the other hand, I tried to add second field as related from first one such as :

x_attribute_line_ids = fields.One2many(related='attribute_line_ids',
                                        invenrse_name='x_product_tmpl_id', 'X Product Attributes')

but it wasn't so accurate.