Many users have changed from a single to dual monitors and their productivity has skyrocketed. I have a laptop that has two screens – one slides out when I need it. The extra real estate can make a big difference especially if you work with several applications open simultaneously.
As inexpensive as monitors are these days, consider treating yourself and your most productive employees to dual monitor systems. Even if you just try a few users first for a trial, you may find why many organizations swear they will never go back to single monitors for their users again.