But the three-nacelled Galaxy class had the hardware of two nacelles in each nacelle housing (three pairs, if you like).
It's still a single nacelle, no matter how many coils or magi-thingies you put into it. And since the housing was identical to the original two nacelles... I think this was just a weak-ass attempt at a retcon for those people who think Gene wasn't being a petty little prat about the popularity of the Technical Manual.