Hi
The software that plays back audio takes one of 2 courses of action
It either uses DirectSound which uses the *default* audio device.
Or it uses the device ID of the hardware to specify the output for the audio.
If it were possible to run two programs side by side to individually specify the separate devices for output, then I would say yes it is possible, but perceived problems would be:
Timing/Accuracy: Audigy 2 card is very accurate, onboard audio isn't anyway near it on a timing basis, so lets say the same music file sets off at the same time on both audio devices, it would be impossible to ensure both audio streams played at exactly the same rate - the program that used both audio devices would have to monitor the throughput of each device and slow/speed one of them up independantly to ensure they were running at the same rate (this is presuming you wanted to play the same audio)
Incompatibility of sound chip - I challenge you to get the Audigy 2 (or any other creative product) to work side by side with another audio chipset - I can't do it, and I don't know anyone that can! - You will find the creative drivers moan/whinge that another audio chip has been detected on the PC and last I tried they refuse to install (which is why you have to disable onboard audio before you attempt to install a creative audio device).
Hope this helps
Nick