Let me start off by telling you a little about my mated pair of Banggais... I bought them approx. 6 months ago; they were just 2 Banggais in a dealer tank with maybe 10 others... I guess I just got "lucky" as I had no idea when I bought them that I had even gotten a male and a female. In fact, until the first spawning, they looked basically identical, except one (the male) was maybe 1/4" longer than the other (the female). They lived happliy in my 125g Mixed reef along with a couple of Tangs, 2 Dwarf Angels, a pair of Oscellaris Clowns, a 6-Line Wrasse and a Yellow - Tailed Damsel.
Here is a pic of my Reef tank from a few months back:
Anyway, I always hoped I might have a pair but was not sure by any means. The 2 Banggais did spend alot of time in the same area of the tank, but were by no means inseparable. I had asked around on how to sex these fish, and was told that the only differences were the length of the second dorsal "thread" (spine?); the males should be longer - and the male should have a "huskier" appearing face compared to the female. Males are also usually a bit larger, but if the 2 fish are not the same age than that is not always an accurate way to sex them.
In my case, the 2 fishes had dorsal threads that looked right around the same length, so I was thinking "both are either males or females"... the slightly larger one did look a little bigger in the face, though.
So I at least had some hope...