OS X El Capitan’s new Split View display mode may not be the most intuitive feature packed into the updated operating system. But once you learn to use it, you'll realise it is the bees knees.
There are two ways to active Split View:
The First way, in my opinion the easier of the two:
You have three dots on the top left corner of the window: red, yellow and green. Red closes the window, yellow minimises it, and green maximises (full-screen) it. Hold down the green button for a few seconds. Once you let you that window will be on one side and you can select the window for the second side.
The Second Way:
Step One: Enter full-screen mode on your first app.
First you have to go to full-screen mode, then merge the apps together. To active full-screen, on the upper left corner of the window you'll see three dots: red, yellow, and green. Red closes the window, yellow minimises it, and green maximises (full-screen) it. Choose the green button to enter full-screen mode.
Step Two: Go to Mission Control.
This can vary depending on how you have your trackpad gestures set up. Either access Mission Control by swiping up on your trackpad with four fingers, or hit the Mission Control key on your keyboard (F3).
Step Three: Drag your second app onto of your first.
Once you've entered Mission Control you should see a all the applications you have open. To enter Split View mode, click and drag a window on top of your pre-existing full-screen app at the top of the screen.
To exit Split View mode simply press the esc key.