sound cards??

I'm not sure if i'm being stupid, but after alot of fiddling around i have finally bought a second usb sound card for headphone cueing, installed a separate driver so it is recognised as a second soundcard by my computer. and now when i go into preferences for Cross DJ it will let me select from a list of soundcards, and it has the option of dual play so i can have the master out of one soundcard and the cue out of the second, but it won't let me select more then one soundcard to use!! even though it clearly recognises i have more then one. the drop down menu for which sound card to use is an either/or option and not both. this seems ridiculous as it means it is physcially impossible to play the master from one output and cue through my headphones in another. making the entire exercise pointless as that is how your supposed to DJ!!! im hoping there is a simple solution as from where im sitting the software looks massively flawed. please help! i just have a laptop with windows 7 and a separate soundsystem running out of the main headphone jack, the idea is to be able to use this set-up in the park for a bit of fun mixing this summer, and so far i have everything i need, except to have the headphone cue come out of a second output channel like it should