I little while back, I wrote an article on using Dual Monitors. In it, I wrote how a person with a visual impairment could use two monitors. Here's a snippet:
"This scenario also works well for those with visual impairments that require a magnification utility. You can use a larger monitor as the ‘main’ display and a smaller monitor as the ‘secondary’ display. Using the main display for all normal functions, the secondary display shows a magnified view of the main display. And without the need for matching monitors, you could pick up a cheap used one to run as secondary. Try your local Goodwill, Savers, thrift shop, etc."
Here's a video showing how to use dual monitors in Windows for users who have impaired vision.