The PM suggestion has come from a study done in the US by several bodies working together and has come up with some interesting results. From memory something like 62% of errors went undetected by the crew and this reduced dramatically when the other guy was nominated to monitor rather than assist.
The article is printed in the July issue of the Business and Comercial Aviation magazine. Unfortunately it is not downloadable so I can not post a link, but it is a good article.
MM