There is no hard and fast rule as there is with Syrians, and different people do things differently. In general, I don’t recommend keeping Chinese hamsters in pairs for new or less experienced Chinese hamster owner, but some Chinese are laid back enough and happily stay together. Generally you're right that males stay together better than females, and I've found my baby girls fall out earlier and more enthusiastically than their brothers. I do know of people who keep female pairs though. Sometimes pairs that stay together during adulthood fall out when they hit about a year old, so it's worth being prepared just in case
Unfortunately Chinese hamsters can fall out with little warning and inflict serious damage on each other. I would therefore recommend that you have a spare cage available and are prepared to have them living separately should they fall out.
From my experience and talking to other breeders, Chinese stay in pairs better if kept in small (cages so neither can establish a territory and without too many toys to fight over.
I have had better success with pairs using lots of substrate, piles of bedding, scatter feeding, some kitchen roll tubes and no houses or wheels. I add toys one at a time from then and monitor for arguing, making sure that any toys have multiple entrances so one ham can't pin another in it. I have found wooden cubes, playsticks and branches good. Deep substrate is good so they can hide from each other if needed. I have followed the approach I've seen in Europe of piling lots of soft hay in on top of the substrate too to break up lines of sight, but do take care if using hay (it can have sharp bits that can damage eyes and pouches, so many owners prefer not to use it with hamsters). Deep substrate is also useful as it reduces the frequency of cleaning and thus the frequency of disturbing the scent in the cage. I would only do partial clean outs, making sure to replace about half of the old substrate to preserve the group scent. Moving from a small cage straight to a large one may precipitate arguments so I make any increase in cage size or arrangement gradually in steps.
Signs to watch for (and check hamsters at each handling for) are uneven patches of fur, bites, plucked areas or one hamster being thinner or smaller than the other. Chinese like to go for the eyes, nose and bottom in bites, though plucking I've mainly seen round the eyes. If there are uneven patches of fur, have a look at the skin underneath as you may find scabs there. If one hamster is getting thinner, the other may be bullying him or preventing him from getting to food (even with scatter feeding). If there is bullying or wounds have been inflicted then you will need to separate. I have a low threashold for separating as serious injury or worse can happed very quickly after just a few little squeaks.
This is an example of plucking of the fur around an eye (yes, it's a robo, but it looks the same in a Chinese!):
Plucked fur by
vectishams, on Flickr