Today, we learned a new technique for miking a guitar amp. It consists of a Shure SM57 on-axis pointing toward the centre of the speaker, and an AKG D112 coincidental with the SM57 but off-axis and also pointing toward the centre of the speaker. Since the D112 is off-axis, it's rejecting higher frequencies. Having both of these recordings together can create a richer and warmer sound.
The two microphones have to be coincidental to avoid any phase issues. Say, the D112 was just a few centimeters further away from the speaker than the SM57 was, this would cancel out a specific frequency since one microphone would pick it up at the wave's peak, and the other at its trough, so when the two signals are summed together the positive and negative charge of these two signals would cancel each other out resulting in silence at that specific frequency.
When the microphones are static, it's almost impossible to notice that any phasing issue exists, however as soon as one microphone is moved forwards of backwards, it becomes incredibly clear. This shows that even if I don't hear an issue after having miked something up with 2+ microphones, there might well actually be one meaning I should always remain vigilent to the relationship microphones have to the positioning of others.