Bi-amping is a process in which a signal is split into two parts using a crossover. The low part of the signal is sent to one amplifier and speaker setup and the high/mid part of the signal is sent to a second amplifier and speaker setup. Since the frequency range of each part of the signal is fixed by the crossover, the speakers (and amplifier) can be chosen for optimum performance in that frequency range (i.e. the low speakers don't have to handle hi/mid frequencies and the high/mid speakers don't handle low frequencies). This leads to more efficient speaker performance and some extra control over the signal, at the expense of a more complex setup.
