im not to sure about this one, but you can configure winamps sounddevice if you are using the nullsoft waveout plugin (choose configure and you will have a selection of avail soundcards).
i dont know how to configure the mediaplayer, but it will only one of the, so configure winamp to use the other
I do however have one question what if you want the change it so the movie sound uses the other soundcard(and ditto for winamp) ???
Were you intending on using it as a fixed system always winamp on one and always movie on the other ??