"If you treat the woofer as a "generator", the signal it sends back to the amplifier is dissipated by the DC resistances along the way (voice coil resistance, cable resistance and source impedance across amplifuer terminals). Just considering the impedances involved, I would think the signal associated with this about as large as a flea fart."
My friend builds speakers for a living and tells me the same thing as John has stated. Any feedback from the low end speaker seems to distorte the high end less by biwiring. I personnally think that it only serves to sell more speaker cable.