well, from what I've read, those ships are designed to move fast in a straight line at warp, but be highly maneuverable at sublight, maybe to warp out to a trouble spot, fight as long as they can, then quickly run away when the bigger ships arrive. The Vulcan ring ships seen on ENT and TNG would also fit this description, The Vulcan's approach to space exploration is more methodical, so they don't need warp maneuverability.
I suppose that if the fighter is only designed to use the engine for short warp jumps, and not FTL combat or interstellar ferry flights, a single engine could make sense.
Have you given any thought to how this thing lands, I can't imagine those wings supporting the weight of the craft on the ground by themselves.