Shutting down the wrong engine in a multi-crew cockpit could lead one to consider slack training. It might be in an atmosphere of 'rushing' and surely would be in an atmosphere of 'not confirming'. The latter can be caused by a training culture. Some QRH's (memory & non-memory) require PM to "confirm" any control which shuts down an engine. There is, of course, a danger of lip-service to this procedure, but, again, that can be trained out in a strong culture. It does seem a staggering statistic and one which should be addressed to find 'out cause'. Mind you, 50% of 2 is 1, so just how many, here, does 50% represent? I admit to not yet having read the whole report.