Multiple sound outputs

Hi... I'm not sure if I'm posting this in the correct section.. but here goes.

I have purchased a small usb sound card for my macbook to set up as a secondary audio output for use with some dj software I have on my macbook Pro.

I basically need the os to see two separate sound outputs on my system.

I want to use the usb sound card as a secondary output for a set of headphones that i want to use to preview tracks.

I have done some research online and believe that it has something to do with setting up an aggregate device, but I can't seem to get the setup correct to make it work.

Any assistance would be appreciated..