Regarding the Kelvin in nuTrek i know of the theory that the ship has two sets of warp coils, just not in separate nacelles but one set inside the other (basically like the different compressor stages in a modern jet engine).
I have heard of that one, too. One of the reasons I feel so uncomfortable with this concept is because it additionally adds to the unpleasant "concept erosion" we already had to endure, IMHO.
- The humanoid TOS FTL ships (U.S.S. Enterprise, Klingon Battlecruiser, "The Deadly Years" Romulan Bird of Prey, the S.S. Aurora) had warp engines spaced noticably apart from each other
- For the TMP Enterprise Andrew Probert felt that the hazardous warp field creation between those nacelles was the major reason to do so, accordingly his Romulan Warbird design features empty space between its warp engines
- Already with the Klingon Bird of Prey (humpbacked warp engines, apparently, unless the wings are warp drive components of some sort) the concept erosion took place and many new TNG designs (e.g. Romulan Scout) no longer followed this rule
Proposing a concept where these warp components no longer need to have space in-between each other for warp field creation (e.g. vessels with single warp nacelle) has an inevitable retroactive continuity effect which I personally find impossible to tolerate:
Why, then, were the warp engines of the TOS (and TMP) ships spaced apart from each other so much?!?!?
This design adds surface space and probably has undesired effects in terms of deflector shield power consumption and defense (wider areas for enemy's targeting).
From a practical point I'm confident that no Starfleet, Klingon or Romulan engineer would have constructed their ships the way they did, unless it was absolutely necessary for optimal FTL performance.