Not so fast. With passive bi-amping, both amps still see the full range of the input signal. Yeah, you get twice the power if you use one more amp like you had before but you don't get the benefits of limiting the bandwidth of the INPUT signal to each amp. This is what active bi-amping is all about and the benefits in that case are considerable.
Now having said that, passive bi-amping can be attractive with some speakers. If the high & low pass sections on the ML's are isolated from each other when configured for bi-wiring, then, during passive bi-amping, each amp will only see 1/2 of the x-over instead of the whole thing. As such, your amps will probably deliver more current to the individual drivers than a single amp would of the same combined wattage looking into the entire x-over. This also means you will have slightly better control over each driver as well.
Also, as is usually the case for electrostatic panels, as the frequency goes down impedance goes up, sometimes considerably. If this is so with the ML's, then the amp would be asked to supply more voltage than current at lower frequencies. This may reduce the stress on the amp running the panels and may be audible. Similarly, as the frequency rises, there is probably an increase in the impedance of the low frequency section. It won't be as dramatic as the rise of the panel, but it too may help to reduce the stress on that amp and may also be audible.
When I passively bi-amped my ML Quests by using another Forte 4a (yes, that amp did a great job with the Quests), I immediately noticed a step up in quality, with all the usual suspects (slam, soundstage, transparency...)going up a notch. The basic charater of the combined amps was the same as a single amp, just noticably more refined and dynamic. The point here being, if you have an amp that you like with your ML's but want more power, just add another amp.
If you do decide to passively bi-amp, one thing you need to pay attention to is to use a HIGH QUALITY splitter (assumming your pre-amp does not have an extra output set)and use identical, high quality interconnects all around.
Good Luck
Now having said that, passive bi-amping can be attractive with some speakers. If the high & low pass sections on the ML's are isolated from each other when configured for bi-wiring, then, during passive bi-amping, each amp will only see 1/2 of the x-over instead of the whole thing. As such, your amps will probably deliver more current to the individual drivers than a single amp would of the same combined wattage looking into the entire x-over. This also means you will have slightly better control over each driver as well.
Also, as is usually the case for electrostatic panels, as the frequency goes down impedance goes up, sometimes considerably. If this is so with the ML's, then the amp would be asked to supply more voltage than current at lower frequencies. This may reduce the stress on the amp running the panels and may be audible. Similarly, as the frequency rises, there is probably an increase in the impedance of the low frequency section. It won't be as dramatic as the rise of the panel, but it too may help to reduce the stress on that amp and may also be audible.
When I passively bi-amped my ML Quests by using another Forte 4a (yes, that amp did a great job with the Quests), I immediately noticed a step up in quality, with all the usual suspects (slam, soundstage, transparency...)going up a notch. The basic charater of the combined amps was the same as a single amp, just noticably more refined and dynamic. The point here being, if you have an amp that you like with your ML's but want more power, just add another amp.
If you do decide to passively bi-amp, one thing you need to pay attention to is to use a HIGH QUALITY splitter (assumming your pre-amp does not have an extra output set)and use identical, high quality interconnects all around.
Good Luck