I'd be remiss if I didn't mention that that Dominion dreadnought was originally done by another modeler. Unfortunately, I don't have a readme included with the model so I I'm not sure who to credit for the model (P-81? Moonraker?). All I did was add the landing skirts in photoshop, to show off the concept for others to ponder...
I would guess that the reasons modelers on this style of DN decided on going with the dual impulse bank design is:
1) The center strut would otherwise go right through the impulse 'crystal'.
2) As the impulse crystal looks cool, modelers like to keep it, so putting them to either side of the strut looks cool
3) You can easily imagine two chambers diverting from the central warp core at 90 degree angles or so to feed the two impulse engine housings. Remember that on the Enterprise-class ships the warp core is a vertical shaft that feeds from the bottom o the secondary hull (where the antimatter is stored) directly vertical to the impulse engines. This is also why the 'third engine' design makes sense, as all you have to do is extend a third warp conduit from the top of the intermix chamber.
This is also way you see two long struts angled to either side of the central impulse engine deck on some other models (thus straddling the impulse deck rather than attaching directly through it). While I think this looks pretty, diverting the warp conduits at 90 degree angles only to bring them back at 45-75 degree angles seems a little impractical. Still looks cool though!
Techically, the saucer section on a Fed DN is supposed to be larger (by about 50% in volume?), with a proportionately smaller bridge section (for those familiar with the SFB model) - as well as a larger, more elongated secondary hull (Firesoul has a model on his site that has an elongated secondary hull, and the saucer appears to have been crafted to reflect the larger 'bulk' of the saucer section as well). However, several modelers found it easier to simply reuse the Enterprise model back then, which is what we see in my pics today. I have since downloaded some other models that have attempted to address this issue.