Well, now that you mention it...
However, the 'shell' of the
T'Pau was meant to be a warp 'ring' while the shell of the
D'Deridex was meant to contain various facilities - the warp engines relegated to the proper nacelles at the side - no doubt inspired by their espionage of Starfleet tech that inspired them to build the 'Balance of Terror' bird of prey and their inspection of Klingon vessels acquired in the 23rd century.
Still, the similarity is a nice bit of subconscious design continuity between the two cousins. Perhaps many other 23rd century and prior Romulan ships employed annular warp drive configurations similar to the
T'Pau? Indeed, I have often wondered how old the
T'Pau was. I could easily see it being of the same age as the Vulcan warp sled from TMP, if not older. Also, perhaps annular warp drive was once a much more common thing than it was in latter-day Trek.