They are very similar gender wise. There are ways to get pairs, however, I think the easiest and most surefire way would be to buy a pair from divers den on live aquaria, they usually have a pair or two every other week.
Youll only be able to have 1 pair of this cardinal. Easiest is to buy an already known pair or buy a few and return the rest once theyve paired off. And its said the males have longer streamers on the first dorsal fin but ive never noticed a difference.
I’ve only ever been able to tell once they pair off and they males carry the eggs. I initially bought four, figuring the odds of them all being either male or female were very low (below 10%). Sure enough, two paired off and I gave the others away.
Males will tend to have a longer second dorsal fin, more asymmetrical "bulldog" looking jaw and broader abdominal fins. But that's not true 100% of the time so I wouldn't rely on that to sex them. As said before, the easiest way to get a pair is to buy a group and let them pair up then remove the rest. As the pair will not tolerate the presence of any other Banggais. But there is another method where you count the vents that are between their abdominal and anal fins. Males have two vents and females have one.