If machinery is simultaneously operated by two or more persons, the employer or user of machinery concerned shall provide such machinery -
(a) at every operation point with a stopping device which locks out when it is used and requiring manual resetting before such machinery can be restarted; and
(b) with an audible warning device to be sounded before the machinery is set in motion: Provided that an inspector may grant written permission for alternative precautionary measures whereby the safety of those persons is ensured.