Bi-amping is about driving the highs and lows independently using an active crossover. With this set up you can use appropriate amplification for both highs and lows for improved efficiency, and less wasted power since the woofer can be driven with an adequately sized amp and an smaller amp can be used for the tweeter.
Passive bi-amping is sending full range signals to the highs and lows and so isn't technically bi-amping. What I've learned is that using identical amplifiers will result in worse efficiency as the amplifiers will limit available power to the woofer by driving the tweeter signal, robbing it, as it were. Since tweeters do not need nearly as much power as the woofer it is a waste of headroom as I understand it.
So it is a very inefficient connection scheme. That is what I've learned about bi-amping. I'm no expert and I've probably left out some details, but for a laymen like me, this is about as far as I'm going to get without cracking open a text book.
Edit : I looked up a few articles on the power distribution of audio signals for the highs and lows and that has given me an even clearer understanding about the whole tweeter/woofer power and why power is wasted with passive bi-amping.