As reported in the NY Times, Jan. 15, 2009, at B9, researchers “recently asked office workers to perform several common tasks using various monitor configurations. They found that people who used two 20-inch monitors were 44 percent more productive at certain text editing operations than people using a single 18-inch monitor.” These days, with flat-screen monitors dropping in price like the Dow and display sizes at 22 inches or larger quite common, the cost is trivial (less than $200) but the productivity gains from dual displays may be significant (See my post of Jan. 18, 2008: multiple monitors; Nov. 9, 2008: energy-consumption concerns; April 27, 2008: flat panels and energy; and Aug. 4, 2008: magnification of screens.).
Everyone is more productive if they can keep two documents fully visible, and cut and paste more easily. Or they can avoid the distraction of switching applications, such as from e-mail to word processing and back (See my post of April 23, 2006: ergonomics; and April 23, 2006: keyboard ergonomics.).