I don't own one nor do I run rekordbox at this time but... The video is going to output from rekordbox running on your computer or laptop. You'll need a video output on your computer or laptop so you can hook up your second screen. That could be a flat screen display or projector. Setup you computer to extend the desktop onto the second screen instead of mirroring the main screen.
I think if you double-click the Preview (MASTER) window in rekordbox it will open an output window with the master video playing in it. Then using your mouse you can drag that output window to your second screen and maximize it / make it full screen.