def add_inline_interface(self, interface_id, second_interface_id,
        logical_interface_ref=None, vlan_id=None, second_vlan_id=None, zone_ref=None,
        second_zone_ref=None, failure_mode='normal', comment=None, **kw):
        """
        Add an inline interface pair. This method is only for IPS or L2FW engine
        types.
        
        :param str interface_id: interface id of first interface
        :param str second_interface_id: second interface pair id
        :param str, href logical_interface_ref: logical interface by href or name
        :param str vlan_id: vlan ID for first interface in pair
        :param str second_vlan_id: vlan ID for second interface in pair
        :param str, href zone_ref: zone reference by name or href for first interface
        :param str, href second_zone_ref: zone reference by nae or href for second interface
        :param str failure_mode: normal or bypass
        :param str comment: optional comment
        :raises EngineCommandFailed: failure creating interface
        :return: None
        """
        interface_spec = {'interface_id': interface_id, 'second_interface_id': second_interface_id,
            'interface': kw.get('interface') if self._engine.type in ('single_fw', 'fw_cluster')
            else 'inline_interface'}
        
        _interface = {'logical_interface_ref': logical_interface_ref,
            'failure_mode': failure_mode, 'zone_ref': zone_ref, 'second_zone_ref': second_zone_ref,
            'comment': comment}
        
        vlan = {'vlan_id': vlan_id, 'second_vlan_id': second_vlan_id}
        
        try:
            inline_id = '{}-{}'.format(interface_id, second_interface_id)
            interface = self._engine.interface.get(inline_id)
            _interface.update(vlan)
            interface_spec.update(interfaces=[_interface])
            interface._add_interface(**interface_spec)
            return interface.update()
            
        except InterfaceNotFound:
            _interface.update(interfaces=[vlan])
            interface_spec.update(_interface)
            interface = Layer2PhysicalInterface(**interface_spec)
            return self._engine.add_interface(interface)