Isso é discutido no Manual Técnico da Star Trek TNG (considerado uma fonte importante de informações sobre o Star Trek universo).
Em suma, diferentes configurações de motor foram testadas no passado, mas para naves acima de um certo tamanho, um projeto de nacele dupla parece ser a maneira mais fácil de alcançar um campo de dobra bem equilibrado, além de oferecer a melhor oportunidade. sair da urdidura de forma controlada em caso de falha catastrófica dos sistemas de voo:
Second, a pair of nacelles is employed to create two balanced, interacting fields for vehicle maneuvers. In 2269, experimental work with single nacelles and more than two nacelles yielded quick confirmation that two was the optimum number for power generation and vehicle control. Spacecraft maneuvers are performed by introducing controlled timing differences in each set of warp coils, thereby modifying the total warp field geometry and resultant ship heading. Yaw motions (XZ plane) are most easily controlled in this manner.
During Saucer Module separation and independent operation of the Battle Section, interactive warp field controller software adjusts the field geometry to fit the altered spacecraft shape. In the event of accidental loss of one or both nacelles, the starship would linearly dissociate, due to the fact that different parts of the structure would be traveling at different warp factors.
Obviamente, vemos naves com designs multi-nacelle, então esta não é uma regra 'dura e rápida', mas oferece um vislumbre no universo da lógica por que vemos essa configuração tão frequentemente em humanos e naves alienígenas.
Fora do universo, isso foi em grande parte uma consideração de design baseada nos próprios preconceitos de Roddenberry. Em uma entrevista de 2005 para o Trekplace Andrew Probert (Concept Artist para Star Trek TOS e ST: The Motion Picture) deu-lhes uma sólida visão geral do processo de design, bem como suas próprias adições ao cânon da técnica:Probert: Gene specified to me, in fact, that starship warp engines operate in pairs... only in pairs because they're codependent. If you had one warp engine, you'd probably go in a circle, I don't know... (laughs) So in the same breath he negated the three-engined dreadnoughts along with the single-engined destroyers, on the edict simply that, to achieve warp drive, you had to have codependent warp engine pairs. As far as the line-of-sight requirement, that was my edict, that, in order to be codependent, the warp engines had to "see" each other, totally. I'm taking about the power combs, not necessarily the Bussard collectors but the bulk of those combs have an energy path between them. And then for other starships, just like in World War II, where all the nations had fighter aircraft that all looked different -- you know, a cultural distinction between, say, a German aircraft and an American aircraft or a Japanese aircraft -- they all operated in the same way having the same basic components of wings, body, and engine, so I applied that thinking to the alien ships I designed as well, so the Ferengi ships, and Romulan Warbirds, have twin warp engines that have to see each other in order to operate. Even my shuttlecraft having a very shallow clearance, still see each other. That's why designs like the Romulan scout ship, where the engines cannot see each other, aren't consistent. There are also some cool Starfleet designs like the Nebula Class ships, but their warp engines cannot see each other. Even those runabouts ignore that ruling which messes up the continuity. Science fiction in particular NEEDS to be consistent. If you negate that,...it all falls apart.
h / t para @HorusKol por me lembrar sobre este site.