Enter Split View
- Hold down the full-screen button in the upper-left corner of a window.
- As you hold the button, the window shrinks and you can drag it to the left or right side of the screen.
- Release the button, then click another window to begin using both windows side by side.
If you already have a window in full-screen view, you can still view it side by side with another window. Just open Mission Control, then drag a window onto the thumbnail of the full-screen app at the top of the screen.
Apps that need more screen space to enter Split View might display a message that they're not available in full screen. You can use a higher resolution on your display to give apps more screen space. Apps that don't support Split View at all show a zoom button instead of a full-screen button .
If holding down the full-screen button doesn't enter Split View, go to Apple menu > System Preferences, click Mission Control, and make sure that “Displays have separate Spaces” is selected.
Work in Split View
In Split View, you can use both apps side by side, without the distraction of other apps.
Original article and pictures take https://support.apple.com/en-us/HT204948 site