its likely they are both males, but i have known an exceptionaly aggressive female or two in my time.
i recently got some javas for the first time in ages, 4 of them. i imediately recognised that it was 3 males and 1 female. the two spare males i took to work and now live in the office. until a few days ago they got on great but now fight like cat and dog. looking at them i realised that one of them was in full breeding condition (the base of the beak becomes swollen and the red colour intesifys) and the other was not, so its a case of territorial behaviour.
leave them to it just now, but if there seems to be any damage, sepperate them if you can. if they simply dont get on, then perhaps swap one for a definate female.