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.
I don't get this.
If they had filled in the space between the wings, they'd had massively increased the amount of space available, and yet not increased the vessel's dimensions. When you have carved out the entire center of your ship, I don't think it's very efficient at all. Federation and Klingon ships at least don't carve big holes through the center of their ships.
Also, given that the only parts of the waarbird that we ever saw with windows are the beak and the horseshoe at the back, it looks like only a fairly small part of the warbird is actually habitable.