You could most probably pick up 2 sound cards and even get them configured without interference, but your problem is going to be that you are going to be unable to use both of them at the same time.
In Windows, though the drivers are loaded, only one of the soundcards will be deemed the sound device and there isn't an easy way of defining which one of the soundcards this is going to be.
If you wish to use four speakers, your best bet is to buy a soundcard that has dual output's..Regards