Fstoppers demonstrates using a Wi-Fi connection and the Air Display app for Apple iPad with a Windows laptop -- it also uses on a Mac.
Fstoppers demonstration uses the Air Display app to connect an Apple iPad via Wi-Fi. If you want to eliminate any lag situations, you might want to use the Duet Display app, which uses a Lightning cable or a 30-pin cable connection to a Mac's USB port. A set up with two Apple iPads used as two external displays is also possible. The setup works with all desktops and notebooks running OS X 10.9 or Windows 7+; and all Apple iPads and iPhones running iOS 7.0+.
Unbox Therapy demonstrates a Triple Monitor Mega MacBook with DuetDisplay and Ten One Mountie Clip.
See also ...
Fstoppers: [Video] Turn Your iPad Into A Wireless Monitor For Dual Displays
Duetdisplay.com
App Store Duet Display