Once you've selected the output soundcards in the Preferences/Recording settings/wave devices dialog box a master channel section will appear on the mixer for every output soundcard each section contains separate return controls for all the auxs, so you can set the return level for an aux channel to -Inf for an output card and to say 0 dB for the other output card. Now you can use the send sliders as they were another regular mixer that goes to the second output soundcard. You may want to activate the option "pre-fader send" to make the sends completely independent from the track main volume and pan.
n-Track | Create Music 