EAX is a dinosour.. if anything, I would (personaly IMHO etc.) try to avoid it.
this is an article covering the EAX and I3DL2 API's.

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EAX Advanced HD is the next generation of 3D audio rendering API and today only a few hardware and software applications support it.

which probably won't change soon, esspecialy because on Vista

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Australia, January 29, 2007 - "Windows Vista and DirectX 10 signal a new era in gaming," so says Mark Walker, Microsoft columnist and gaming evangelist. And he's but one of a chorus of voices from within and without Microsoft touting the merits of the next generation of Windows when it comes to gaming. So imagine your surprise when you fire up one of your favourite games in Vista - say World of Warcraft or Prey - only to find your fancy EAX-endowed soundcard and 5.1 surround speakers are dribbling out flat, unenhanced stereo sound. Then, in a vain attempt to spruce up the audio by enabling EAX, you get a nice taut error message saying EAX is not detected on your hardware. What's going on? Welcome to the world of Vista audio. And a brave new world it is.

the option now open for gamers who can't live without these effects in games that were developed to use them is the Open AL, also described in the 2'nd link.

for simplicity's sake, I wouldn't go there if I'm not a hardcore gamer (I"m not so that's again just an opinion).
about multiple audio cards, it's not flawless, and again, I think you need to have a specific reason to configure your system that way. otherwise, loading and using 2 different drivers is questionable.