Parallel Audio Playback

I want to configure my computer so that I can watch a video and hear the audio on headphones plugged into my onboard sound card and on my USB headset. However, I can’t find a program that will only output to one sound card at a time. I’ve done some searching, but I can’t find any information on how to do this. Are there any programs out there that will let me output to more than one sound card at a time? Is there a way to relay output from one sound card to another?

Maybe ask this question on IRC freenode channel #alsa and see if any of the sound guru’s there have an answer? I typically use the program “xchat” for accessing IRC.