I'm new here, so please bear with me

At home I have set up a gaming rig and a sound-studio combined.. I am using an external soundcard + pre-amp for recording and stuff...
I have ordered a ButtKicker and it's on its way. But all the tutorials have confused me.. How can I dedicate my external soundcard to X-Sim, and at the same time hear the sound in my earphones? Do I have to dedicate one of my soundcard to X-Sim, and at the same time use the other soundcard for the sound to my earphones? How can I make those two run at the same time?
I tried to see if there was an option in X-sim to "choose" my dedicated soundcard, but didn't find any options for that.. I'm a tad confised

-Kim R