Maybe the wrong term, but I want to use a NAD 7100 as a monoblock. I know I can bridge it with a 2100 amp to power each channel independently, but since the "guts" of the 7100 are essentially the same amp with the pre-amp and tuner added to it I wondered if I could use the amp section only of a second 7100.
There's no need to duplicate functions like tuner and pre amp on the monoblock when they are not necessary. If you can bridge the receiver into a monoblock amplifier and you're certain the two amplifiers sound alike with equal power output, then you can do what you are proposing.
What are you expecting to gain from this? The bridged amp will double its wattage. Doubling wattage alone doesn't really gain much.