I believe I've seen this on an upper level Yamaha. It's not nonsense. The unit is put into 5.1, which liberates two amp channels. Those channels are used as the extra channels needed for biamping. The fact that the amps were originally used for back channels just means that the internal inputs to the power amps are swapped around to accomplish this.
I'm wondering whether the line level signal is available before power amplification so that the one major advantage of biamping can be used: an adjustable active crossover.
A good answer is easier with a clear question giving the make and model of everything. "The biggest problem in communication is the illusion that it has taken place." -- G. “Bernie” Shaw
