Passive biamplification questions

You should contact the manufacturer to see if there are any potential problems with bi-amping. Some amplifiers go into oscillation with some speakers in biamp mode.

Will

You didn't say if you were using identical amps.

Using identical amps vertically will not increase perceivable power much since most of the the power is used by the woofers. However, the drivers can interact with the amp and with each other. Biamping reduces that interaction. For twice the amps, don't expect twice as good.

Different types and models of amps have various strengths and weaknesses. For example, I enjoyed the Monarchy SE100 for mids, highs and imaging but they lacked bass. My previous amp lacked detail but was only a little shy of a nuclear bomb on dynamics. Biamping those together, I got the "best" of both worlds, eventually. Unfortunately, those amps did not have the same sensitivity or gain and it wasn't a happy marriage without some electronic counselling.

An attenuator on the louder amp is DIY project beyond many. Using an active X-over along with passive X-over in speaker is possible but tricky. Removing the speaker's passive X-over and replacing it with an active risks turning the speakers into junk or can have fantastic results. There are a couple digital X-over/ preamps available that can do everything except grate cheese.

All this is based on a low crossover point (80 to 120 Hz). Above that will get into critical midbass where subtleties aren't so subtle.

If one is using two class AB or A/AB amps the more efficient drivers which are usually the higher frequency drivers which our ears are more sensitive to may have more class A amplification available to them which may sound better. IMHO more often than not and the specific cross-overs may have a lot to do with this, using identical amps is prefered.