Bi-wiring is running two sets of wires from a single amp to a speaker's two sets of posts.

Passive bi-amping involves driving each set of binding posts with a separate amplifier channel. If both amps are identical, then the two posts are going to get the same magnitude signal - which corresponds to the way the speaker manufacturer designed the internal crossovers. After all, the two binding posts would get the same magnitude signal if they were jumpered together and connected to a single amp by a single cable. If you use different amps for the high and low, you would potentially need to balance the two signals so they are equal when they reach the speaker.

Active bi-amping involves ripping the crossover circuits out of the speaker, building new crossover networks located upstream of the two amplifier channels, and then amplifying the output of each crossover and passing that directly to the drivers inside the speaker cabinet. That particular approach is not one that is adviseable for the average person because of the level of knowledge needed in crossover circuit design and speaker design.