In most general discussions, passive biamping refers to using the speakers internal crossover vs an external crossover - although there are external passive crossovers too.
Vertical biamping refers to using the left and right outputs from one amp driving their own section of one speaker.
Horizontal biamping refers to using the channels of one amp for the lows of the left and right speaker, and then another amp to drive the mid/highs of the left and right speakers.
Don't worry about the power, you are making much ado about nothing. The cardinal rules are turn it down if the speakers start making strange noises, or if the sound takes on a harsh and or flat, non-dynamic sound.