NAD amps can be bridged, but when I did so with my 3400 and 2400 amps (sister products) to rum my Mission 765's, there was a very definite decline in the sound quality. The trade-off to maximizing power output(and there is always a trade-off) is the sound output took on a much harder edge. Bi-amping preserves all of the sonic/headroom benefits with absolutely none of the trade-offs.
In your case, I would have used one of the amps (the one with independent level control) for the HF and the other for the low; or the more powerful amp for the LF.