marine bettas have no known sexual dimorphism. The most common way to attempt a pairing with these fish is to introduce a small group into a large aquarium and monitor behavior. Males are said to be intolerant of other males, while they can live and breed with one or more females. The aggresion you are seeing could be the result of having 2 males, but it could also be a result of the established fish protecting it's territory. They are demersal spawners, and have been breed succesfully, but they are an un-common species to attempt. I have not bred them, so the 411 I'm posting here is just a combination of stuff I've read from other people who have bred them as well as my own experience in pairing clowns...which normally wouldn't apply as I try and pair clowns at a young age, before they reach sexual maturity and one changes sex to female. But when pairing adul clowns, even when I know their genders, there is a decent chance they will go to war if I just throw one in with the other. The best way I've found to reduce this risk is to move both specimens into a new environment/tank at the same time. Could be true for bettas as well. Anyway if you want to breed them, you will probably end up spending a LOT of time doing research. I hope this helps!
-M