In Windows 8.1, you can work on two tasks by displaying two apps on one screen.
On the Start screen, run the app you want.
Move the cursor to the top center of the running app screen.
When you drag the cursor down, the screen is reduced in size.
While moving the smaller app screen to the left or right, a vertical control bar appears.
After placing the app on the left or right side of the screen, release the left mouse button to fix the app to that position.
Click on the empty screen to go to the Start screen.
When you select another app in Desktop or on the Start screen, the app is displayed in the empty half of the screen.
You may change the screen ratio of an app by using the vertical control bar on the screen.
When you drag the vertical control bar to the end of the outer edge, the opened app screen disappears.
You can split the screen by also clicking the app icon in the title bar at the bottom-right of the app that is running.