A/B setups allow you to optimise for different tones. Fender amp for clean and a Marshall for crunch for instance.

As for a pair of 2x12" vs. one 4x12", a lot depends on the speakers. Two identical amps running together will only sound a bit louder than one amp. Hearing is a logarithmic function, and doubling the power does [u]not[/u] double tha apparent loudness. The increase would be 3dB, which is commonly accepted as the minimum readily perceptible loudness change. If the 4x12" cab had more effficient speakers in it, one amp could be much louder than a pair of 2x12" combos.

Horizontal placement vs. stacks: Speakers placed near the floor will propagate low frequecies better. And take up more floor space. Speakers placed at the player's ear level will produce hearing problems in said player. Either way, the sound man will probably insist on micing just one speaker, and shutting the rest off.