Comment There actually seem to be some studies (Score 3, Informative) 208
Some googling turned up a description of 3 studies (the fourth actually doesn't look at dual monitors)
http://www.kevinkane.com/2010/12/4-studies-which-show-that-using-a-second-monitor-can-boost-productivity/
Maybe not high quality research, but they all show an increase in productivity due to a second monitor