Bi-amping does not effectively double the power. Problem being is that somewhere around 65-80% of power is actually soaked up by the woofers. You can say you have double the power and technically that is correct but the potential will never be realized or manifested as doubling of power since neither the mids nor the tweeters will ever dissipate the same power as the woofers.
However, bi-amping does provide more headroom for a cleaner reproduction of transients since the rail voltage does not sag as much from say thunderous bass passages.
If one amp is driving all the drivers and the rail voltage sags a bit from transients, well, it does take some "fraction" of a second to recover but the music doesn't wait. The soundstage starts to collapse and the sound can and does get a bit congested. That's way before you get to clipping.
With two amps and one of them working hard, it leaves the other amplifier to drive the mids/tweets with clean power reserves. Not to mention there is much less chance of blowing your drivers if only one amplifier is driving and clipping from too much enthusiasm and does not offer protection from DC clipping.
Technically the Horizontal bi-amping makes sense due to the reasons I mentioned above. The woofers are making the amp work hard and sag a bit but the other amplifier does not see that load.
With vertical biamping the whole amplifier still can sag from the woofers in that channel and has to recover which would affect the whole speaker unless the amplifier is truly dual mono with separate power supplies. That's not common for a stereo amp.
By far the most profound and efficient way to do bi or tri amping is with an active system. Problem being you would have to totally bypass the crossovers so the the best case scenario is a "system". I'm not very familiar with Bryston active systems but do they not offer something along those lines?
