Second screen mode allows you to open your hosts secondary display.
In order for the second screen option to appear in your Parsec overlay, your host computer needs two displays connected, with the screens configured to "Extend these displays" mode inside of Windows settings. You will not see the option if the host screens are set to "Mirrored" or "Duplicate" mode.
In order to change the resolution of the second screen, please do this from the Windows Display settings menu.
You can change the bandwidth, resolution, chroma settings, and frame rates per screen at any time. That makes it easy to have one screen set up perfectly for your IDE while the other is set up for gameplay.
Parsec currently supports up to two screens.