Check out "Bi-Amping the Denon 3805" and "Tripp Lite Isolation Transformer" threads for more discussion on bi-amping.
In a nutshell, the truly "correct" (i.e. pro audio) way to bi- or tri- amp is with an active crossover upstream from your amps and bypassing the passive crossovers in your speakers. Unfortunately that probably means opening up your speaker cabinets to disable the passive x-over (check w/ your speaker mfr for more info).
You can do it with full-range amps and the passive x-overs in your speakers, but you won't get the full benefit of bi-/tri-amping, which is to let each amp channel work with only one driver on a limited range of frequencies. As far as the amps can tell, they're each trying to drive the whole speaker from 20 to 20k and the passive x-over is dissipating the power to the frequencies outside the range of each driver.