Reply to Newbie here

Hey Guys

My problem is that I can't figure out how to set my traktor PRO demo to work with my headphones.
I got an Imac to which I connected my Numark Stealth, Also I've connected my Alesis io2 express to the computer.
To the IO2 i've connected my alesis speakers and my headphones.
When in the traktor preferences I chose the IO2 as the main output sound.
And here the problem, I can't listen to song A in the monitors and song B in the headphones and then cue them.
Any Idea on how to solve this? or if i'm doing something wrong?