How old are they? It sounds like they have hit puberty! Young rabbits often use a box fine, but when they start getting their hormones litter box habits go out the window.

You should plan to get them neutered, and box habits will return.  Here’s some more basic info on litter box training: https://binkybunny.com/infocategory/litterbox-training/

Also, if you have them living together, you should plan to separate them soon. Young rabbits get along, but puberty will cause them to start fighting in many cases. Once they are neutered, you can go through the bonding process with them.  (read about bonding here: https://binkybunny.com/infocategory/bonding/)

Also, young rabbits are very often mis-sexed, and you could end up with an accidental litter.