Hello,
Sorry to hear you are having issues. Not really sure, your issue. Usually, if you want to apply a background or a playlist of backgrounds. You will click on Apply playlist or background->The apply menu open up and you if you wish to set to specific monitor. You will click on "To a Specific monitor", yes?. This will then open up the drop down menu with list of monitors available, yes? If you choose one and it open up to the wrong monitor, try the other one on the list. That should apply to the other one which the one you need, no?

If these steps are not what you get. Perhaps, I misunderstood you, it happen to me quite a lots recently. Post a video showing us what you get. That will help us understand it better.

Thank you,
Basj,
Stardock Community Assistant.