Steve Albini walks through his approach to miking multi-speaker guitar cabinets, covering both the logic behind using multiple microphones and the practical decisions that shape the sound you capture.
When a cabinet has two speakers, Albini places a ribbon on one and a condenser on the other. The ribbon reads darker and heavier, the condenser brighter and more present. Recording them separately gives you flexibility in the mix, and it opens up the option of synthesizing a stereo image from a mono source.
He also adds a distance mic on the floor to capture room ambience. Placing it on the floor shifts its pickup pattern from omnidirectional to hemispherical, so it collects reflections from the walls and ceiling while avoiding near-field floor reflections that would color the sound.
For combo amps with two speakers, like a Vox AC30 or Fender Twin, Albini explains the concept of the phantom center: the point between the two speakers where their sound sums into a single coherent source. You find it by crouching in front of the cab and slowly moving your head side to side until everything locks into one point. That's where a single mic belongs if you want the full character of the amp in one placement.