Are they properly bonded? This means 1) they are both fixed and 2) they have gone through the bonding process (prebonding, dates, supervised monitoring…). Hormones change rabbits drastically, regardless if the rabbits have been together since birth or are from the same family. So my initial thought is your rabbits may not be fixed and are now acting on hormones.
If they are not bonded and you would like to have them both, they will need to go through the bonding process or they may seriously hurt each other. They may seem happy sometimes, but in the end, an unhealhy bond is very dangerous.
The first step would be to separate them from each other for their safety. Two separate pens/habitats.
Are they both fixed? How long have they been together? And how old are they?
The answers provided in this discussion are for general guideline purposes only. The information is not intended to diagnose or treat your pet. Seek the advice of your veterinarian or a qualified behaviorist.