Personally, I've viewed the dual-hull design of the D'deridex-class warbird to be influenced more by politics than by any propulsion system. I think the Romulans wanted to build the biggest and most intimidating starship they could and chose a design that was huge in the sense that it had an enormous length, beam, and draft, but due largely to a giant hole in the center.
Actually, when you think it through, the Romulans made much more
efficient use of space than the Feds. Had they left the upper or lower "wing" out of their design, the ship would have looked much like an average Federation or perhaps Klingon warship; now there is much more useful volume in the ship for the given outer dimensions.