It sounds like the degus in the question here are just playing, especially if neither is getting injured in these "battles." Usually serious fighting is pretty hard to mistake for play, both by its intensity and the fact that serious fighting often results in injuries. Here are some guidelines for determining whether fighting is play, a minor issue, or a serious problem that requires intervention.
Play Fighting
Degus will often engage in play behavior which may involve a degree of play fighting: boxing, chasing, and nipping. They will often chatter at each other as well. This sort of activity is often mutual -- the degus will take turns chasing each other, for example. Injuries are rarely seen as a result of play fighting, and the degus involved usually get along the rest of the time, sharing food and toys and cuddling together to sleep.
Minor Squabbles
Sometimes, degus will have minor squabble over a treasured resource. This could be food, special treats, favorite toys, etc. These disagreements are usually quite minor, but if you think your degus are having these sorts of squabbles too often, you can try adding additional food dishes and toys, or perhaps a bigger cage. As with play fighting, the involved degus usually get along well the rest of time and snuggle together when sleeping.
Dominance Struggles
Degus need to establish a social structure (dominance hierarchy) so must determine who will be "top degu" (alpha degu). This is often established fairly easily, but when degus are quite evenly matched for age, size, and strength, it may be more of a struggle and more fighting may be seen. This fighting may be a bit more intense than play fighting, and may involve some growling, grunting, tail thumping and kicking, especially with evenly matched degus. However, even in this case, the battle is usually settled before serious fighting or injuries occur (though the occasional small scratch or scrape may occur). Dominance behavior can also involve the more dominant degu mounting the other.
Once dominance struggles are settled, the degus usually calm down and go back to getting along, playing and sleeping together.
Struggles over dominance may start out as minor squabbles, but progress to more serious fighting, so monitor the situation if fights seem to escalate in frequency or intensity.
Serious Fighting
Serious fighting usually starts out with threatening grunts or growls and tail thumping, and progressed to wrestling, biting, and kicking at each other. The degus may roll together in a ball and really bite at one another, especially around the head and neck. Usually, the vocalizations and biting are much more intense than with any kind of play fighting or minor squabbling. If your degus are fighting like this or really injuring one another, intervention is necessary.
Separating Fighting Degus
Leather gloves are recommended to prevent getting bitten yourself. Throw a towel over the degus to distract them and make separating them easier, then use your gloved hands to remove one the offending degus. Place the degus in separate cages, at least temporarily.
Degus that have fought this seriously may never get along, so it may be best to simply house them in separate cages. Some owners will try to re-introduce degus that have had a serious fight. This involves having two cages, and swapping the degus back and forth between the cages to get them used to each other's scent and decrease the territorial instincts involved. However, if two such degus are re-introduced, you must be prepared to watch them carefully and separate them again if they go back to fighting. If they fight a second time, it is best to separate them permanently.