Because the Windows 8 experience is split between new Windows 8 Store apps and old-school desktop apps, the operating system is prone to some strange behaviors. for example, when running a multi-monitor setup, Windows 8 apps will consume your main screen, leaving your secondary screen running the desktop. This arrangement would seem to allow full-screen multitasking among both types of apps,a modern app on the left side, a desktop app on the right side, but this isn't the case. Indeed, as soon as you begin using the desktop on your secondary screen, the new-style Windows 8 app disappears, and your primary screen begins running the desktop.
But here is a workaround. On your primary screen, use the new Windows 8 split-screen snapping function to run the desktop and a new Windows 8 Store app together. The desktop can take up the left-hand sliver, while the Windows 8 app consumes the majority of the screen. Now use your second display for a full desktop view. In this arrangement, you can fully multi-task between new-style apps and desktop apps, and both windows will be large enough to be useful.