I've thought about that while watching them all. But not just flaring, but I think there's a form of communication beyond us going on. When I had the two boys in the split 10g, Kalec would be on the farside, faced away from the divider with plants in the way - but EVRY time Seiya got close to the divider, Kalec would zoomie over and immediately stsrt flaring out. A threat, but the way he knew without seeing makes me wonder about their hearing. Lateral lines sensing vibration is my first theory.
But it opened up the idea to me of how they cpmmunicate beyond threat displays or when mating.
So i find it very interesting how your girls did that and what it might mean.