In and article over at Datamation the answer seemed to be that it depends on the person and the job maybe.  If you are a developer there is evidence that you can improve your effectiveness by twenty to thirty percent with a second screen.  While this article isn’t very scientific it does give those in operations or management a nice insight into what people are thinking.  It does make for interesting question about whether developers should have laptops.  Check it out and see what you think.