In reply to:
In that case, assuming that the higher frequency speakers share no connection with the lower frequency speakers and that they're normally wired in parallel, the impedance the amps would see would be some multiple of the original. However, exactly what that multiple is would be dependant upon the individual cones and the other components involved.
What Semi described above is correct only if both the mid/tweeter and woofer terminals of the speaker present a purely resistive load that is completely flat over the audio frequencies. But that is far from reality in most cases. For example, the impedance across the woofer terminals on a bi-wirable speaker usually gets progressively higher above the low-pass crossover frequency, because of the serial inductor in the crossover circuit. The same is true for the mid/tweeter terminals, and the impedance progressively increases below the high-pass crossover frequency due to the serial capacitor.
So, in the single-amp configuration where these terminals are connected together in paralell. the woofer (and mid/tweeter) port becomes progressively "invisible" to the amp above (and below) the crossover frequency, respectively. And hence, in the bi-amp configuration, the minimal impedances across the individual mid/tweeter and woofer terminals (which occur at totally different frequencies) are usually not much higher than the minimal impedance when they are connected in parallel.
