Click the 
icon on the toolbar.
This will open the Output Configuration
dialog box.
Make sure that your main soundcard is selected in the "ON-AIR" output device and that Enabled option is selected.
Click on Set next to the "CUE" output device section to select your second soundcard.
Select your second soundcard drivers and click on OK.
Make sure that the Enabled option for the "CUE" output device is selected.
Close the Output configuration dialog box.
To test, play your favorite Ots files.
Select the Mixer by clicking
twice on the 
button on the toolbar.
Click the Cue button. You will notice the cue channel output through your second soundcard.
Note: If you have received the "Failed to allocate audio device. Close other audio programs and start again" error message, this could be because you are trying to enable the same soundcard in the both the "ON-AIR" and "CUE" output options. You can not allocate the same device in both places.
Note: Old School mode cannot be used across two soundcards.
Tip: You may notice the ON-AIR signal through the cue channel. For instructions on how to remove the "ON-AIR" signal from the cue channel click here.
How to remove "ON-AIR" signal from the cue channel
How to set up your soundcard (main channel)
Output Configuration dialog box