Hi Lou
With boys it can be a testosterone thing (think teenage boys with hormones running amok
) but it can be just plain old sibling rivalry where you've got two pups the same age, same sex and with personalities which are just too similar so they rub each other up the wrong way. Things can be fine when the pups are young - generally problems will only develop from about 8 or 9 months when mentally dogs are changing from puppies to adults. That's not to say that litter mates cannot get on - a dog & a bitch from the same litter are more likely to get on than 2 brothers for example. Also if there are complimentary personalities, things will probably be fine eg if you've got one placid, submissive type pup and one pushier pup, then the risk of the dogs not getting on is far less than if you've got two pushy pups
We might very occasionally let go two pups go to the same home if the owners are very experienced with lots of time on their hands but we'd never let two brothers go the same home - it's just not worth the risk of it going wrong. We also wouldn't keep two brothers ourselves for the same reasons
Opinions differ on when exactly the time is right to get a second puppy but the general consensus is that it should be at least a year later but 18 months to 2 years is even better. A lot does depend on the temperament of the 2 dogs though as I know we have plenty of members on COL with 2 dogs close in age who have no problems at all
Jane