You don't have to bother now. What's important is that you (and all newbs reading this) now realize that what you originally claimed cannot be done.
Having turned this over in my brain a few times, I'm beginning to think maybe that this is in fact theoretically impossible (as in, according to modern theory, it can't be done).
To sum up the idea, it's based on the principle that any audible signal can be effectively considered to be a sum of pure sinusoid signals in varying proportions. Combining two signals (like a mixed stereo file) is simply further summing of sinusoid signals (correct me if I've got this wrong).
Based on that, a combination of signals "doesn't care" where any given sinusoid amplitude originated in the original signals. It only "cares" about the total description of the combined sinusoids.
This is why filtering a signal to remove a noise is often difficult: when you drop the appropriate frequencies of the signal, you can't specify to only remove the gain values that "belong" to the unwanted signal. The mixed signal doesn't care about that distinction. So filtering at a particular frequency to remove noise doesn't really remove noise, it just makes that entire sinusoid component less loud.
It might seem like this isn't the case because you can do things like inverse summation for null testing. However, in that case, the filtering is trivial because you know exactly which frequency elements to attenuate and by what amplitude to perform said attenuation. This is why when you "null out" a signal, you don't hear artifacts of it: the signal doesn't actually "care" where the original summation came from, and nullifying simply removes the elements of that signal. In the real world, you need to have a complete description of the individual elements of the mixed signal to separate a stereo file in the way boblybob is proposing. But if you've got that complete picture, you don't need to separate because that means you've already got the individual elements (at least as they're presented in the stereo field).
Does any of that makes sense? Am I way off-base on that?