If you want both channels output through a single speaker rated at 8 ohms, you could just connect it conventionally via one set of speaer terminals, then set NAD's speaker impedance selector to 8 ohms, and then press the "Mono" selector button on the front right panel of the receiver, to merge stereo signals into a single mono output.
Do you actually need to double the power output, anyway?
Do you actually need to double the power output, anyway?