If these are degus, who haven't lived together yet, then try to let them live in two cages standing right next to each other for a while. After some time try to let them together and if they fight, separate them again and repeat that.

To let them together is better on some open place, not in closed cage. The reason is that in the cage weaker degu has no chance to run away and he could get hurt. In nature degus don't hurt each other because of dominancy fights, weaker degu just runs away. Do it on some place where you can let degus run out, like in a hall or in bedroom (where you closed all the openings and the toilet). There put cage on the ground and open it, so that degus can freely run out and back inside. Put there both cages and let them make things clear, who will be the boss between them. Some argues and little fights are normal, just be prepared to separate them if they fight too hard. In that case don't put your hand between them, they could bite you by accident.

You can repeat this, until you lost hope that they will calm down someday. Some degus need more time and then they become friends, others can't live together no matter how long you try.

Sometimes unfortunately even degus who lived together, even a year or so, start fighting. Then act the same way - separate them and try to put them together after some time, and if they fight, then do it again. Sometimes after a short separation, another time after few tries, they stop fighting. But sometimes they still fight and they can't live together anymore. The reason mostly that degus at the age around one year (when this happens mostly) became full adults and their dominance relationships change. Then also degus who have been 'weaker' fight for the place of the 'boss' int he cage. Mostly some time of separation helps.