Sometimes one single display is not enough, so many professionals in different areas tend to install two or more displays in their system.
DuetDisplay lets you establish a connection between your main system and a portable device, such as an Android tablet, an iPad or a Windows-based device, This way, you can have an extra display and, for example, use your iPad or Android device as a drawing tablet for your desktop PC.
The program can also allow you to access remotely to your desktop, which is another of its advantages.
Another advantage of this system is that it does not require any hardware such as cables or dongles. You only need to install it in the devices you want to join together and connect to the Internet in case of the remote desktop access.
A disadvantage I found is that Windows cannot verify the publisher of some of the drivers needed to work with different devices.
Please note that, although the Windows program can be downloaded and used for free, the apps for Android or iOS are cost-based. As you need the program to be installed on both devices, you will have to purchase the corresponding app.