Don_s...In a 2-way speaker system of,say, 8 ohms, each of the drivers will be 8 ohms. True, the drivers are in parallel, but because of the crossover the power amp "sees" the woofer alone at low frequency, and the tweeter alone at high frequency, so the overall impedance remains 8 ohms.
Biamping does increase the effective power of the amp. If you do not biamp the HF signal is riding on (superimposed on) the LF signal. The peak signal, which is what may cause the amp to distort, will be the sum of the two, If you biamp properly using an electronic crossover, each amp has only the HF or LF to deliver. The HF, for example, starts from zero, rather than where the LF signal is, and therefore has further to go before distortion results.
This power gain was important back in the days when 20 watt amps were the rule, but today it is simpler to just get a bigger amp. Also, in the old days, intermodulation distortion (which occurs when a HF and a LF are amplified together) was typically 1 percent or higher, and biamping helped. But today's amps have lower IM.
Today biamping remains useful, and almost universally used, for the subwoofer/main speaker crossover, because the inductors and capacitors needed for frequencies below 100 Hz are large and very expensive. The only advantage of biamping the main speaker is to avoid the passive crossover. Makes you wonder why some people simply hook up two amps without using an electronic crossover, and bypassing the built-in passive crossover.