They behave like a pair... the littler one is just coming of age, so I'll probably know if they're a pair pretty soon, LOL. To observe dorsals I have to get close enough that they hide. They still haven't forgiven me for my long and protracted attempts to get them out of their original tank.
These fish are wonderful and not very aggressive! i got three from Becca a few months back and their purple color is very unique .. I keep the sexes in different tanks because they will breed...