An application can only use one card at a time, as far as I know.
I have on-board and another internal soundcard. To choose which an appliction will use; right click on the speaker icon in the system tray and click on Adjust Audio Properties. On the Audio tab, select which sound card for playback, apply, then launch an application. You can then change which sound card for the next application.
This way it's possible to set up for Teamspeak/Ventrillo in your headphones, and LFS through your speakers, for example.