How to Split Screen / View in Mac OS

OS X El Capitan or later macOS lets you fill your Mac screen with two apps by the Split View Mode

Enter Split View Mode

1. Hold down the full-screen button  in the upper-left corner of a window. 
2. As you hold the button, the window shrinks and you can drag it to the left or right side of the screen.
3. Release the button, then click another window to begin using both windows side by side.

Inside Split View Mode You can...

• Swap window positions by dragging a window to the other side.
• Adjust window width by dragging the vertical line between the windows.

Note:

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.

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.

Exit Split View Mode

1. Move the pointer to the top of the screen to show the window buttons, then click the full-screen button .
2. The other window remains in full-screen view. You can switch to it with Mission Control, or use a Multi-Touch gesture such as swiping left or right with four fingers on your trackpad.

Source : Use two Mac apps side by side in Split View - Apple Support