The way offering least complication on the night would be to have your loop as a video set to play on the first slide (enable repeat). The next click of the presenter would take you into your presentation proper.
OR in Windows (alter shortcuts to suit for your OS )
If both presentation were already full screen, and you were running Windows, you could Alt+Tab into the other presentation. You would need to be on the ball as you would have to Alt+Tab into the slideshow, not the editing window.
Better to use two virtual desktops (Task View > New desktop Win+Tab) each with a presentation in full screen sideshow. To switch between them in Windows use Ctrl+Win+arrow. New desktop and shortcuts
I suspect there are other ways too. Cheers, Al