Can't really be done - biamping (v1) depends on an integrated to provide the signal to the tweeter, and a power amp (fed by the integrated's preouts) to power the woofers. You could bridge the power amps to mono and use one for each speaker though (biamping v2).
I wouldn't go splitting the signal from the preamp to the power amps, you could run into all sorts of problems. There are a couple of preamps out there (IIRC) that have two sets of variable gain preouts to do just this, but your NAD ain't one of them.