I think many people get confused, probably rightly, by the words that often don't describe things properly. Two-channel and stereo are used as having the exact same meaning. They both use two paths between mics and recording equipment then inside it too. Stereo bolts the two channels together and locks them to create a soundfield that when listened to on two speakers, left and right, recreates the sense that you can close your eyes, and point to certain features. Like maybe with a string quartet, where the viola player is sitting , and with a drum kit perhaps a soundfield that puts you in the drummers seat, or more commonly just standing in front. Stereo piano lets you hear where on the keyboard hands are, and if you are on the pianist's side or the audience's. Stereo guitars in a deadish room sound mono if you are listening from more than a couple of metres. They are a point source. If you mic them up with two mics to get the fingers on the fretboard, and the fingers on the sound hole, then the result is not stereo, it's two channel, and you can eq each separately, and adjust the level and even effects on both to create something new. That isn't stereo. Novices often mess up stereo from the point of realism. A grand piano might be big, but if your 'stereo' technique produces an instrument that uses all the space between a listeners speakers, that's plain uncomfortable to listen to. Listening to a Jerry Lewis piano recording would be plain horrible, like watching ping pong at the Olympics, if they miked that up as left and right, panned hard. Can you picture how odd that would sound? So stereo recording should be dealt with on a purpose point of view. Stereo means what? Sounds like you are there on the front row, or there as in actually on stage? Perspective is everything. You have a recording, and you have the power, if you recorded it appropriately, to make it useful to a blind person, so they can picture where everything really was without their eyes. Equally, you can produce something totally wild that could never have existed practically. Both can legitimately be stereo. For me, my version of stereo is realism as if I was in that front row. Other people disagree and want something different. That's the end product. Up until that mix, I think I'd call everything twin track for accuracy, because the stereo effect doesn't exist at that stage. A/B or X/Y and others are legitimate stereo pairs of signal paths, but then convention is to always treat both legs the same to retain the realism. X/Y on drum overheads as I saw somebody do last week in a live show just fights with the panning of toms, snare and hats, wrecking the stereo X/Y can produce, on its own! What I do know is that proper stereo recording that works is very hard, probably tougher to get right than a multitrack.
