You'd be better off with optical. The only thing analogue will do is let you play the lossless tracks and I can near assure you, that you won't be able to tell the difference. With optical you'll get the DD and DTS tracks which are practically transparent from source. I'm also sure that the DACS in your amp will be a lot better than those on the X-Fi and one cable > many. You wouldn't get high resolution audio as that requires hardware that support the protected audio path, though it's a gimmick anyway.
But as to your question it's a 3.5mm jack/cable just like the regualr stereo output. How do you think you can get stereo from one jack? Same thing, the jack has rings for each channel, you must have noticed that if one isn't inserted properly you don't get sound from both channels.