If you mean putting Windows side by side, what you need to do is clicking and holding green light button in apps capable to run in full screen mode. Then, there should be a Place to let you choose another app capable of doing so. If you choose one of them, you will see how multitasking works.
Here is another way to do this: bring an app to full screen, and go back to desktop, click and hold title bar of a window, and quickly drag it to the top of your screen, where you menu bar locates. Then, you should be able to know, you can put this app with previous app, which is entered into full screen mode, side by side.
If this is not, It would be glad if you can specify.
Regards.
白崎つぐみ