Step 1: In the 'Window' menu choose 'Fullscreen Options' to open the Output Options window.
Step 2: Click the 'Fullscreen' tab and check the boxes for the Displays you want to use.
Tip: If the height and width of the canvas for your project are not the same aspect ratio as your connected displays, use the Sizing Mode to determine how the canvas is resized for output.
Step 3: To exit Fullscreen mode: Click the 'Window' tab.
The keyboard shortcut to enter / exit Fullscreen mode is ⬆+⌘+ f