Your USB headset is, in fact, an USB soundcard and a headset. And you cannot output sound on two soundcards simultaneously...
Even so there could be a way doing this if both soundcards were based on Creative chips. Here you can find a powerful driver and mixer that allows outputting sound from two soundcards:
http://kxproject.lugosoft.com/index.php?skip=1. It will work with your Soundblaster, but I’m not sure about the headset...