Don't the speakers have their own crossover?
Most speakers have passive crossovers and these filter circuits are not compatible with a connection to more than one amplifier, hence the use of only two connectors for each particular speaker. Other speakers that allow bi-amping have four binding posts, two for each amp. Their internal passive filters are wired separately and that's what enables the bi-amping possibility.
For most home audio systems, bi-amping is not required. If the amplifier doesn't have enough power output, it's most of the time cheaper to use a more powerful amp.
In my situation, I bi-amp the 3 front channels because the required crossover frequency between the enclosed sub and the 2 mid-range drivers is fixed at 200 Hz. At such low frequency with a passive crossover, the series inductor resistance would affect the transient response and the efficiency of the subwoofer.
