In general, you shouldn't discourage puppies from playing fight. It is completely normal and desirable behavior in young dogs up to several months of age. Puppies fight with their litter mates, mother and friendly adult dogs to develop their skills, body coordination and strength control. Puppies play with their littermates constantly.
From about two weeks, when they open their eyes, until they go to their own homes, they spend almost all of their waking hours fighting each other. It's a critical time for social development because that's when they learn to inhibit bites and have good manners with dogs. It's good exercise and socialization for them and fun for us to watch. But you must learn to differentiate between playing and a real fight when adult dogs are involved.
If they see each other during training sessions, they might notice each other's bad habits or, worse, not pay attention to them at all. This takes time and probably a lot of patience. Like children, puppies need time to develop their own personality and good habits. They do it best with positive reinforcement.
Placing two puppies in the same household always made at least one puppy temperamentally unfit for the job, even when both puppies started out as perfect candidates. Training two puppies of different breeds and ages together can be a real challenge, especially if they are from the same litter. We must say that there is no conclusive scientific evidence of littermate syndrome, but the owners have shared story after story about their experiences of adopting two dogs at once, how what they thought would be a great idea failed. Professional trainers recommend not having two puppies six months apart, because the risks are too high.
In addition to the issues one might expect when bringing siblings home, such as double food and vet costs and double potty training work, new puppy owners should focus on how puppies will develop. Puppy brains continue to develop until they reach sexual maturity (and even a little beyond that), and there is some convincing research that says that bringing two puppies home at the same time prevents one or both of them from reaching their full potential. My brother and I (we live in the same house) adopted Rottweiler puppies named Bach and Beethoven (both males) and for the past few days, it seemed that they were having fun playing together until recently they started fighting hard, where both my brother and I were scared that one of them might get hurt. This doesn't even take into account the other practical considerations, such as the increased costs of veterinary care, food, supplies, and training; the extra work of training and caring for two dogs; or the time needs of two active puppies.
This instinct of an adult dog to play fight with puppies in an old one, deeply embedded in its DNA from the days when they were wild. Willing households received not one, but two puppies to breed, thus doubling the number of puppies the guide dog organization could work with. Puppies need to spend more one-on-one time with their new owners than they have with each other, effectively doubling the work and denying any of the potential benefits (i.