One of our readers posted a comment that should be expanded upon. Vertical vs. horizontal bi-amping. The terms are rather simple, yet they are important.
Vertical bi-amping is a practice I touched on yesterday–one channel of a stereo amplifier for the tweeter, the other for the woofer. Horizontal bi-amping is dedicating both channels of one amp to the woofer, and both channels of a second amp to the tweeter.
Vertical bi-amping is valued because the majority of the power supply of one amp is dedicated to powering just the woofer. This is because tweeters take next to no power. It's probably a good compromise when two amps are used and the way I would recommend if you're using identical amplifiers.
Horizontal bi-amping is valuable when you're using different amplifiers to optimize sound quality–brutes on the woofer, sweet and small on the top end.
So we could make a rule of thumb that suggests vertical is best for identical amplifier models, horizontal is best for optimizing the synergy of different amps.
But what if you want to go "balls against the wall" crazy? Then use four monoblocks.