Microsoft want to enhance multi monitor support and aim to:


• Make the desktop a more personal experience. Perhaps the most personalized feature on the desktop is the ability to customize the desktop background. We set out to make this a great experience on multiple monitors too.

• Improve the efficiency of accessing apps across monitors. In Windows 7, the top request from people using multiple monitors was to improve the taskbar efficiency.

• Improve the efficiency of accessing system UI. In Windows 7, you could only access the Start menu on one monitor. With the introduction in Windows 8 of new UI that puts controls at the edges of the screen, we wanted to make sure that it’s still easy to access Start, the charms, the clock, and your recently used apps from every monitor.

• Allow side-by-side Metro style and desktop apps. You can launch or move a Metro style app to any monitor, side-by-side with desktop apps on another screen.
Taken from the latest 'Building Windows 8' blog, the full article can be found here:

Enhancing Windows 8 for multiple monitors - Building Windows 8 - Site Home - MSDN Blogs