I used to have a Soundblaster card that had multiple inputs and it was cool for what I used it for.
I guess the only way you can get multiple inputs is to split the stereo line-in, in to two channels but they both would then be mono. Apart from that, as BatVink says, extra hardware and software is what you need.
Using two sound cards may give you the extra input channel too but I can't be too sure about recording though although I can't see why not.
Mental arithmetic? Me? (That's for computers) I can't subtract a fart from a plate of beans!
Warning! May contain Nuts!