One of El Capitan’s new features is Split View, which similar to features introduced in Windows 7 allows you to quickly set an app to full screen in half your display’s viewing area. With this approach, you can utilize maximal screen real estate to quickly work on two separate windows from different apps. This mode should be easy to access, but after installing El Capitan you may find Split View will not work.
By default, you should be able to click and hold the green “fullscreen” button on most application windows and documents, and when the blue hue overlays the screen, you can drag either to the left or right to place the app along the desired side of the screen. You can then select another app to appear on the opposite side, and then get to work.
Both of these approaches should be doable on any system running El Capitan; however, if they are not working, then it is very likely you had previously used your Mac in a multi-monitor environment, and subsequently adjusted the setting for each display to have its own desktop space (done in the Mission Control system preferences).
Having separate spaces for multiple displays is the default screen-handling approach for OS X Yosemite, and is required for using Split View. Therefore, simply check this option, followed by logging out and back into your account, and you should see Split View work as expected.