It's never been stated in any episode or movie, but the concept behind nacelles is that they work in concert with each other to generate a warp field (hence why there's always more than one) and supposedly require line of sight to work with each other. Andy Probert is a firm believer in this premise. When someone showed him an Eaglemoss model of the Nebula class, he pointed out that the nacelles should stick out lower, since the secondary hull is in the way. (We can only imagine what he thinks of the Defiant). I suppose you could argue that Voyager's pivoting nacelles, allowing them line of sight, is consistent with this. As I said, this has never been stated on screen and a few other ship designs do break that "rule". (The Jenolan, the Buran, and possibly some of the "First Contact" ships, the runabout, and possibly some other shuttles, for example). Do you consider it canon or not? Sidenote: Probert hated the future Enterprise-D from "All Good Things", partyl because it had three nacelles, but I can't see any reason why three of them couldn't work in concert with each other.