Hi,
Have you ever had these birds DNA sexed? Usually when lovebirds fight, the female is more likely the aggressor. It’s possible the aggressive bird is ready to breed and the other one isn’t. Lovebird aggression should be taken seriously, so I would separate them for now. If you planned to breed them, they really should be about 2 years old before you have them together. If one is ready and the other isn’t, the more mature bird might kill the reluctant one. If you have not tested these birds and are just guessing their sex, I would get them DNA sexed. You could even have two of one sex. Even if they are male and female, it doesn’t mean they will be compatible as a breeding pair. As long as they are fighting, they should not be caged together. Harmless sparring is one thing, but when it turns into chasing and not allowing the other to eat, it tends to escalate to feathers being pulled out or blood being drawn and finally one bird gets killed by the other. I would not cage them together and definitely not set them up to breed until you are sure you have a male & female, and they are getting along.
Thank you for asking Lafeber,
Brenda