we weren't talking about drums. as I said before, in relation to drums, I'm NOT for sliding those tracks because of the loss of phase coherence between the 8-12 microphones I use for that. we're talking about guitar amps, and in that application, assuming you are trying to combine the signals of two mics that are supposed to be coincident (as bryan k was shooting for in his original question). we aren't talking about combining a distance mic and a close mic (or at least, I'm not).
We all have to keep in mind that when a microphone picks up a sound, it picks up all 360 degrees of every cycle. The mic that is further back picks up the exact same sound (assuming it isn't so far back as to significantly change the ratio of direct sound to reflected sound) a fraction of a second later. THEREFORE, if you were to move that track back in time in relation to the reference track, they would HAVE to be in phase.
The phase of a sound does not change in any significant way over the distances we are using. Look at my other posts.
jimmyjazz...yeah...that's what I'm getting at with drums. I'd just like to stick to the original topic because I think it's a valid discussion, and as far as I can tell, I've got a pretty good handle on the situation, however, I'm really interested to see what JJ brings back in terms of illustrations.