...but would you need two soundcards or is it possible with only one?
No, you won't need more than one sound-card. My headset is a USB set. It is recognized by TS, Ventrillo, Windows Sound configuration, and other multitudes of programs that allow me to select which input/output device to use.
Not sure how it work, but I don't care, I like it. +1 
I normally use my 2.1 for music and Headset for game, but it would be nice to have the sounds on the speakers instead. A P38 rumbling by on the Sub-woofer sounds great with volume up. 
Uhm, from what I understand you have a 2.1 setup and you have headphones plugged into it? This would mean that your speakers don't play audio, instead your headphones play audio until you unplug them. That wouldn't work with what we're asking for. What we're saying we have is a setup similar to this:
1 USB Headset (Microphone and speakers)
1 Sound system in the form of two speakers or more
And we want to be able to choose that our speakers play back audio of a certain type, while our headset plays back a completely different form of audio.
Your setup (as I gather) is one sound system with two speakers or more with headphones attached to the speaker-system. This setup can't be altered because the headphones attached to your speakers are still detected as just... speakers.