In the case of passive biamping, i.e., biamping without an electronic crossover "ahead" of the power amps, it is of course correct that what has to be matched in some manner is the gain of the two amps (gain being the ratio of an amplifier’s output voltage to its input voltage, for a given load impedance), not their maximum power ratings.
In the context of a passive crosover, while it seems as a simple thing to just match the gain of the two "different" amplifiers used in biamping, it is much more than that. Two different amplifiers will never have the same timing and phase response in the music reproduction. So in effect you are feeding two differently timed signals to the two drivers (LF and HF) and also both signals vary in their respective phasi-ness. The final output will be a sound where the drivers do not integrate and sing as one. The HF and LF will be all there but they will sound like individuals. All the work done at the crossover to marry the drivers are nearly lost. The same happens when biwiring with different cables for LF and HF.