With "passive" biamping each amp must swing the peak-to-peak voltage of the full range signal, so, as Almarg says, no significant power increase will be realized.
On the other hand, when you use an electronic crossover (real biamping) at a given total signal level, the peak-to-peak voltage swing of each amp is reduced. The high frequency signal does not ride on the Low, and vice versa.
Some increase in total power will be realized.
However, the greatest benefit of real biamping is the reduction of IM distortion. This was more important years ago when amps commonly exhibited more than 1 percent IM distortion.
On the other hand, when you use an electronic crossover (real biamping) at a given total signal level, the peak-to-peak voltage swing of each amp is reduced. The high frequency signal does not ride on the Low, and vice versa.
Some increase in total power will be realized.
However, the greatest benefit of real biamping is the reduction of IM distortion. This was more important years ago when amps commonly exhibited more than 1 percent IM distortion.