When your degus fight, your first instinct will always be to stop them. If they are making a Chittering sound, resist that urge as they're just playfighting. This is a natural occurrence among degus and can mean dominance attempts or just simple playing. Degus make other noises while playfighting too, to let the others know what is going on, and you will probably hear one of the degus complain. This usually means they're not interested in playing, and it lets the other degu know...if he doesn't heed the warning, a simple NO from you should suffice to stop it, although you may need to do it a few times and be at the cage and right up in their face for them to realise you're talking to you.
If two degus are on their hind legs with their paws at each other or holding each other, this is known as boxing. Degus may or may not chitter while doing so, but it is entirely safe for them unless you can clearly see they are actually fighting and causing pain.