You can choose either “Mirrored” or “Extended Desktop” mode for your displays. In Mirrored mode, you have a single desktop and it’s mirrored on all your displays. In Extended Desktop mode, each display simply gives you more desktop room and they’re separate.

You can also toggle between Mirrored and Extended Desktop modes from anywhere in Chrome OS via a keyboard shortcut. Just press the Ctrl and (F4) keys at the same time.