How to make two monitors act as one on a Mac.
The ability to have two or more external monitors connected to your Mac in Extended Mode is a liberating screen experience, but leads to frustration when each still operates like an independent display.
Here we show you how to make the two Extended screens act like one with a few simple changes to your system settings.
Without this solution, you can’t have a browser page or application window shared across both screens. Instead, the window is hidden on one of the screens and only partially seen on the other.
It’s all to do with the Mac Spaces feature that lets you create multiple “desktops” and organize your apps and windows so they’re visible on separate desktops, instead of all on the same desktop.
Read more at MacWorld.com