“If you were stationed on Thalassa, you’d see Naiad passing above and below in a pattern that would repeat itself every four loops, as Naiad repeatedly laps its neighbour.”
It’s almost like one moon orbits the other. A moon’s moon.
According to the picture, it appears that Naiad always has a slightly shorter radius orbit than Thalassa so they never collide. Just coincidence? Doubting that.