Oh, sushi, you seem to be indicating that he meant the impedance that each amp would see. He was so unclear in his original post, that I thought he was talking about the speaker.
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.
Loads in a parallel structure add like this:
1/Rt = 1/R1 + 1/R2 + ... + 1/Rn
