Bi-amp is not a menu option like in Arcam receivers, but you can wire pre-outs to amp inputs to bi-amp the NAD 773
Most, if not all, NAD receivers have the power amps connected to the pre-amps by links between phono/RCA type sockets on the rear panel. This enables the pre-amp outputs to be fed into an active crossover which can then feed the inbuilt and/or external power amps. Surprisingly, I cannot find any details on the internet of anyone who has actually done this with a NAD receiver. Practical accounts of bi-amping using any sort of equipment seem rare. I am thinking of doing this myself, so if anyone knows of any links which give details of actual experience, rather than theory, I would like to know.