So when I am fully cross faded to the right (or left deck) and want to cue/preveiw an upcoming track on an opposite deck in my headphones, the cued track plays through the main speakers over top of what's being played live to the main (club).

What setting do I have wrong? Apologies if I have overlooked the answer, but I tried to self help first. Probably a good question to have and have answered in the forum.