1. Open the Desktop by clicking Desktop tile on the Start Screen.
2. Next, Right click on your desktop and select Screen Resolution.
3. Next, in this Display Settings box, you can set up multiple monitors. Click on the display showing 1 to set first monitor and 2 to set the second monitor. You may change the orientation, Resolution over here. You can also make the selected monitor primary by checking ‘Make this my main display’. After you are done click on the Apply button and then OK. That are the complete steps to set up dual monitors in Windows 8.