Aggression in bristlenoses is most commonly between males. However, they are heavy waste producers, and I think 2 might be pushing the filtration system of a 15 gallon. I would limit myself to 1, and have 3 male guppies instead, and maybe an extra khuulie loach; they are happier in groups. Even better might be to choose between a larger group of male guppies and a larger group of khuulies.