Extended desktop in Linux

My goal is to have an extended desktop like in windows. Linux only uses my second monitor as a mirror, which is useless. I have searched a lot on google and SO for a way to get this right. I want linux to use both my monitors for one extended desktop, not as a mirror. As of yet I have not found out how to do this. Any help appreciated, thanks.

What is extended desktop?

With extended desktop mode, you can get a larger Windows* desktop area spanning multiple displays. You can move applications from one display to the other. All Intel® graphics products support extended desktop mode across two displays.

What is the difference between duplicate desktop and extend desktop?

Duplicated – The desktop is duplicated (including resolution) on more than one display. This setup can be useful for running demos and presentations. Extended – The desktop is extended across multiple displays. Each extended display has its own unique desktop area (including screen resolution and refresh rate).