Tucker explicitly says there are two, one in each nacelle. Apparently not quite at the centerline of the warp coils, because that would seriously hinder operations... Assuming that there's plasma pulsing at the center, like in the TNG design. Probably just below the coils, then, judging by Tucker's computer display. We never quite learn whether it was the port or starboard nacelle, though.
Our heroes clearly chose to rig just one of the two catwalks for storm cellar, perhaps because that took up less resources and time. It may also have been psychologically the right thing to do.